Explanation
The best method to report continuous assessment of a cloud provider's services to the Cloud Security Alliance (CSA) is through a set of dedicated application programming interfaces (APIs). According to the CSA website1, the STAR Continuous program is a component of the STAR certification that allows cloud service providers to validate their security posture on an ongoing basis. The STAR Continuous program leverages a set of APIs that can integrate with the cloud provider's existing tools and processes, such as security information and event management (SIEM), governance, risk management, and compliance (GRC), or continuous monitoring systems. The APIs enable the cloud provider to collect, analyze, and report security-related data to the CSA STAR registry in near real-time. The APIs also allow the CSA to verify the data and provide feedback to the cloud provider and the customers. The STAR Continuous program aims to provide more transparency, assurance, and trust in the cloud ecosystem by enabling continuous visibility into the security performance of cloud services.
The other methods listed are not suitable for reporting continuous assessment of a cloud provider's services to the CSA. The Cloud Controls Matrix (CCM) assessment by a third-party auditor on a periodic basis is part of the STAR Certification Level 2 program, which provides a point-in-time validation of the cloud provider's security controls. However, this method does not provide continuous assessment or reporting, as it only occurs once every 12 or 24 months2. The tools selected by the third-party auditor may vary depending on the scope, criteria, and methodology of the audit, and they may not be compatible or consistent with the CSA's standards and frameworks. Moreover, the tools may not be able to report the audit results to the CSA STAR registry automatically or frequently. The SOC 2 Type 2 attestation is an independent audit report that evaluates the cloud provider's security controls based on the American Institute of Certified Public Accountants (AICPA) Trust Services Criteria. However, this report is not specific to cloud computing and does not cover all aspects of the CCM. Furthermore, this report is not intended to be shared publicly or reported to the CSA STAR registry3.
References:
STAR Continuous | CSA
STAR Certification | CSA
SOC 2 vs CSA STAR: Which One Should You Choose?

The control audit frequency for an organization's business continuity management and operational resilience strategy should be conducted annually. This frequency is considered appropriate for most organizations to ensure that their business continuity plans and operational resilience strategies remain effective and up-to-date with the current risk landscape. Conducting these audits annually aligns with the best practices of reviewing and updating business continuity plans to adapt to new threats, changes in the business environment, and lessons learned from past incidents. Reference = The annual audit frequency is supported by industry standards and guidelines that emphasize the importance of regular reviews to maintain operational resilience. These include resources from professional bodies and industry groups that outline the need for periodic assessments to ensure the effectiveness of business continuity and resilience strategies