The CSA Cloud Controls Matrix (CCM) is the best tool to perform cloud security control audits, as it is a cybersecurity control framework for cloud computing that is aligned to the CSA best practices and is considered the de-facto standard for cloud security and privacy1. The CCM provides a set of 197 control objectives that are structured in 17 domains covering all key aspects of cloud technology, such as identity and access management, data security, encryption and key management, business continuity and disaster recovery, audit assurance and compliance, and risk management1. The CCM also maps the controls to various industry-accepted security standards, regulations, and control frameworks, such as ISO 27001/27002/27017/27018, NIST SP 800-53, PCI DSS, GDPR, and others1. The CCM can be used as a tool for the systematic assessment of a cloud implementation, and provides guidance on which security controls should be implemented by which actor within the cloud supply chain1. The CCM also includes the Consensus Assessment Initiative Questionnaire (CAIQ), which provides a set of "yes or no" questions based on the security controls in the CCM that can be used to assess a cloud service provider2.
The other options are not the best tools to perform cloud security control audits, as they are either not specific to cloud computing or not comprehensive enough. GDPR is a regulation that aims to protect the personal data and privacy of individuals in the European Union and the European Economic Area3, but it does not provide a framework for cloud security controls. FIPS 140-2 is a standard that specifies the security requirements for cryptographic modules used by federal agencies in the United States, but it does not cover other aspects of cloud security. ISO 27001 is a standard that specifies the requirements for establishing, implementing, maintaining and continually improving an information security management system within the context of the organization, but it does not provide specific guidance for cloud services. Reference:
Cloud Controls Matrix (CCM) - CSA
Cloud Controls Matrix and CAIQ v4 | CSA - Cloud Security Alliance
General Data Protection Regulation - Wikipedia
[FIPS 140-2 - Wikipedia]
[ISO/IEC 27001:2013]

The most important audit scope document when conducting a review of a cloud service provider is the document that defines the processes and systems to be audited. This document should clearly identify the objectives, criteria, and boundaries of the audit, as well as the roles and responsibilities of the audit team and the cloud service provider. The document should also specify the scope of the cloud service provider's services, such as the service model, deployment model, geographic location, data classification, and compliance requirements. The document should also describe the scope of the audit evidence, such as the types, sources, methods, and sampling techniques of data collection and analysis. The document should also state the expected deliverables, timelines, and reporting formats of the audit. The document should be agreed upon by both parties before the audit commences.
The document that defines the processes and systems to be audited is essential for ensuring that the audit is relevant, reliable, consistent, and complete. It helps to establish a common understanding and expectation between the auditor and the auditee, as well as to avoid any misunderstandings or conflicts during or after the audit. It also helps to focus the audit on the key risks and controls related to the cloud service provider's operations and performance. It also helps to ensure that the audit complies with the applicable standards, frameworks, and regulations.
Reference:
Cloud Audits and Compliance: What You Need To Know - Linford & Company LLP How to audit the cloud | ICAEW Auditing Cloud Computing: A Security and Privacy Guide

Establishing ownership and accountability most enhances the internal stakeholder decision-making process for the remediation of risks identified from an organization's cloud compliance program. Cloud compliance refers to the principle that cloud-delivered systems must comply with the standards required by their customers. Compliance requirements may include data protection regulations such as HIPAA, PCI DSS, GDPR, ISO/IEC 27001, NIST, and SOX. A cloud compliance program is a set of policies, procedures, and controls that help an organization to achieve and maintain compliance with these requirements12.
A cloud compliance program involves identifying, assessing, prioritizing, and mitigating the risks associated with using cloud services. To effectively manage these risks, an organization needs to establish ownership and accountability for each risk and its remediation. Ownership and accountability mean assigning clear roles and responsibilities to the internal stakeholders who are involved in the cloud compliance program, such as the cloud service provider, the cloud customer, the cloud users, the cloud auditors, and the cloud regulators. By doing so, an organization can ensure that the internal stakeholders have the authority, resources, and incentives to make timely and informed decisions for the remediation of risks123.
The other options are not the most effective ways to enhance the internal stakeholder decision-making process for the remediation of risks. Option A, automating risk monitoring and reporting processes, is a good practice for improving the efficiency and accuracy of the cloud compliance program, but it does not address the issue of who is responsible for making decisions based on the monitoring and reporting results. Option B, reporting emerging threats to senior stakeholders, is a good practice for increasing the awareness and visibility of the cloud compliance program, but it does not address the issue of how to prioritize and respond to the emerging threats. Option D, monitoring key risk indicators (KRIs) for multi-cloud environments, is a good practice for measuring and tracking the performance and effectiveness of the cloud compliance program, but it does not address the issue of how to align and coordinate the decisions across different cloud environments123. Reference := Cloud Compliance Frameworks: What You Need to Know1 Cloud Compliance: What It Is + 8 Best Practices for Improving It2 Cloud Computing: Auditing Challenges - ISACA

The greatest risk associated with hidden interdependencies between cloud services is the lack of visibility over the cloud service providers' supply chain. Hidden interdependencies are the complex and often unknown relationships and dependencies between different cloud services, providers, sub-providers, and customers. These interdependencies can create challenges and risks for the security, availability, performance, and compliance of the cloud services and data. For example, a failure or breach in one cloud service can affect other cloud services that depend on it, or a change in one cloud provider's policy or contract can impact other cloud providers or customers that rely on it.12 The lack of visibility over the cloud service providers' supply chain means that the customers do not have enough information or control over how their cloud services and data are delivered, managed, and protected by the providers and their sub-providers. This can expose the customers to various threats and vulnerabilities, such as data breaches, data loss, service outages, compliance violations, legal disputes, or contractual conflicts. The customers may also face difficulties in monitoring, auditing, or verifying the security and compliance status of their cloud services and data across the supply chain. Therefore, it is important for the customers to understand the hidden interdependencies between cloud services and to establish clear and transparent agreements with their cloud providers and sub-providers regarding their roles, responsibilities, expectations, and obligations.3

Shadow IT refers to the use of IT resources (hardware, software, or cloud services) within an organization without the explicit approval of the IT or governance team. This practice is often flagged in cloud audits due to potential risks of compliance violations and security threats. The CCAK documentation from ISACA highlights the need for visibility and governance over all IT assets, with specific controls listed in the CSA CCM for Cloud Governance (GOV-09). Shadow IT poses risks to data security, compliance, and can introduce vulnerabilities, as systems are not subject to organizational standards and oversight.