The optimal and most efficient mechanism to assess the controls that the provider is responsible for is to review third-party audit reports. Third-party audit reports are independent and objective assessments of the provider's security, compliance, and performance, conducted by qualified and reputable auditors. Third-party audit reports can provide assurance and evidence that the provider meets the industry standards and best practices, as well as the contractual and legal obligations with the SaaS company. Third-party audit reports can also cover a wide range of controls, such as data security, encryption, identity and access management, incident response, disaster recovery, and service level agreements. Some examples of third-party audit reports are ISO 27001 certification, SOC 1/2/3 reports, CSA STAR certification, and FedRAMP authorization123.
Reviewing the provider's published questionnaires (A) may not be optimal or efficient, as the published questionnaires may not be comprehensive or up-to-date, and may not reflect the actual state of the provider's controls. The published questionnaires may also be biased or inaccurate, as they are produced by the provider themselves.
Directly auditing the provider © may not be feasible or necessary, as the independent contractor may not have access to the provider's environment or data, and may not have the authority or expertise to conduct such an audit. The independent contractor should rely on the third-party audit reports and certifications to assess the provider's compliance with relevant standards and regulations.
Sending a supplier questionnaire to the provider (D) may not be optimal or efficient, as the supplier questionnaire may not cover all the aspects of the provider's controls, and may not provide sufficient evidence or assurance of the provider's security maturity. The supplier questionnaire may also take a long time to complete and verify, and may not be consistent with the industry standards and best practices. Reference := How to Evaluate Cloud Service Provider Security (Checklist) Cloud service review process - Cloud Adoption Framework How to choose a cloud service provider | Microsoft Azure

During the implementation phase of a cloud assurance program, the focus is on establishing the operational aspects that will ensure the ongoing security and compliance of the cloud environment. This includes developing the monitoring goals and requirements which are essential for setting up the assurance framework. It involves determining what needs to be monitored, how it should be monitored, and the metrics that will be used to measure compliance and performance.
Reference = The information aligns with best practices for cloud migration and assurance programs as outlined in various resources, including the Cloud Assurance Program Guide by Microsoft Cybersecurity1, which discusses the importance of developing and implementing policies for cloud data and system migration, and the Enterprise Guide to Successful Cloud Adoption by New Relic2, which emphasizes the role of observability in cloud migration, including the establishment of monitoring goals.

The management review of the information security framework is an activity that typically occurs outside the regular scope of information security monitoring. This review is a strategic exercise that involves evaluating the overall direction, effectiveness, and alignment of the information security program with the organization's objectives and risk appetite. It is more about governance and ensuring that the security framework is up-to-date and capable of protecting the organization against current and emerging threats. This contrasts with the operational nature of security monitoring, which focuses on the day-to-day oversight of security controls and the detection of security events.
Reference = The answer provided is based on general knowledge of information security practices and the typical separation between strategic management activities and operational monitoring tasks. Direct references from the Cloud Auditing Knowledge (CCAK) documents and related resources by ISACA and the Cloud Security Alliance (CSA) are not included here, as my current capabilities do not allow me to access or verify content from external documents or websites. However, the concept of separating strategic management reviews from operational monitoring is a well-established practice in information security management.

Metrics around Platform as a Service (PaaS) development environments are frequently immature, as PaaS is a relatively new and evolving cloud service model that offers various tools and platforms for developing, testing, deploying, and managing cloud applications. PaaS metrics are often not well-defined, standardized, or consistent across different providers and platforms, and may not capture the full value and performance of PaaS services. PaaS metrics may also be difficult to measure, monitor, and compare, as they depend on various factors, such as the type, complexity, and quality of the applications, the level of customization and integration, the usage patterns and demand, and the security and compliance requirements. Therefore, PaaS metrics may not provide sufficient insight or assurance to cloud customers and auditors on the effectiveness, efficiency, reliability, and security of PaaS services12.
Reference:
Cloud Computing Service Metrics Description - NIST
Cloud KPIs You Need to Measure Success - VMware Blogs

Risk appetite and budget constraints have the most substantial impact on how aggressive or conservative the cloud approach of an organization will be. Risk appetite is the amount and type of risk that an organization is willing to accept in pursuit of its objectives. Budget constraints are the limitations on the financial resources that an organization can allocate to its cloud initiatives. Both factors influence the organization's strategic decisions on which cloud service models, deployment models, providers, and solutions to adopt, as well as the level of security, compliance, and performance to achieve. An organization with a high risk appetite and a large budget may opt for a more aggressive cloud approach, such as moving critical applications and data to a public cloud provider, while an organization with a low risk appetite and a small budget may opt for a more conservative cloud approach, such as keeping sensitive information on-premises or using a private cloud provider12.
Reference:
ISACA, Certificate of Cloud Auditing Knowledge (CCAK) Study Guide, 2021, p. 17-18.
CSA, Cloud Controls Matrix (CCM) v4.0, 2021, p. 63.