Explanation
The correct answer is A. To providing a standardized approach to security and risk assessment. This is the main purpose of FedRAMP, which is a government-wide program that promotes the adoption of secure cloud services across the federal government. FedRAMP provides a standardized methodology for assessing, authorizing, and monitoring the security of cloud products and services, and enables agencies to leverage the security assessments of cloud service providers (CSPs) that have been approved by FedRAMP. FedRAMP also establishes a baseline set of security controls for cloud computing, based on NIST SP 800-53, and provides guidance and templates for implementing and documenting the controls1.
The other options are incorrect because:
B: To provide agencies of the federal government a dedicated tool to certify Authority to Operate (ATO): FedRAMP does not provide a tool to certify ATO, but rather a process to obtain a provisional ATO (P-ATO) from the Joint Authorization Board (JAB) or an agency ATO from a federal agency. ATO is the official management decision given by a senior official to authorize operation of an information system and to explicitly accept the risk to agency operations, agency assets, or individuals based on the implementation of an agreed-upon set of security controls2.
C: To enable 3PAOs to perform independent security assessments of cloud service providers: FedRAMP does not enable 3PAOs to perform independent security assessments of CSPs, but rather requires CSPs to use 3PAOs for conducting independent security assessments as part of the FedRAMP process. 3PAOs are independent entities that have been accredited by FedRAMP to perform initial and periodic security assessments of CSPs' systems and provide evidence of compliance with FedRAMP requirements3.
D: To publish a comprehensive and official framework for the secure implementation of controls for cloud security: FedRAMP does not publish a comprehensive and official framework for the secure implementation of controls for cloud security, but rather adopts and adapts the existing framework of NIST SP 800-53, which provides a catalog of security and privacy controls for federal information systems and organizations. FedRAMP tailors the NIST SP 800-53 controls to provide a subset of controls that are specific to cloud computing, and categorizes them into low, moderate, and high impact levels based on FIPS 1994.
References:
Learn What FedRAMP is All About | FedRAMP | FedRAMP.gov
Guide for Applying the Risk Management Framework to Federal Information Systems - NIST Third Party Assessment Organizations (3PAO) | FedRAMP.gov Security and Privacy Controls for Federal Information Systems and Organizations - NIST

Explanation
The Cloud Controls Matrix (CCM) is a cybersecurity control framework for cloud computing that can be used by cloud customers to build an operational cloud risk management program. The CCM provides guidance on which security controls should be implemented by which actor within the cloud supply chain, and maps the controls to industry-accepted security standards, regulations, and frameworks. The CCM can help cloud customers to assess the security posture of their cloud service providers, document their own responsibilities and requirements, and establish a baseline for cloud security assurance and compliance. References := Cloud Controls Matrix (CCM) - CSA1 What is the Cloud Controls Matrix (CCM)? - Cloud Security Alliance2 Certificate of Cloud Auditing Knowledge (CCAK) Study Guide, Chapter 5: Cloud Assurance Frameworks

Explanation
According to the ISACA Cloud Auditing Knowledge Certificate Study Guide, the auditor should review the relationship between the cloud service provider and its service provider to help direct and estimate the level of effort and analysis the auditor should apply1. The auditor should understand the nature and scope of the services provided by the service provider, the contractual obligations and service level agreements, the security and compliance requirements, and the monitoring and reporting mechanisms. The auditor should also assess the risks and controls associated with the service provider, and determine if additional audit procedures are needed to obtain sufficient assurance.
The other options are not the best approach for the auditor. Option A is too strict and might not be feasible or necessary, depending on the type and level of services provided by the service provider. Option C is too lax and might overlook significant risks and gaps in the cloud service. Option D is too narrow and might ignore the impact of the service provider on the cloud customer's business context. References:
ISACA Cloud Auditing Knowledge Certificate Study Guide, page 13-14.