The suggested answer is C. External audit.
Reasoning: External audits are conducted by independent third parties, providing an objective assessment of the CSP's compliance with the shared responsibility model, relevant standards, and regulations. This objectivity is crucial for customers to verify the CSP's claims and ensure that the CSP is fulfilling its responsibilities.
An external audit provides a level of assurance and transparency that internal audits or security assessments may not offer, as they are conducted by entities with a vested interest.
Reasons for not choosing other options:

A. Contractual agreement: While the contractual agreement outlines the responsibilities, it doesn't provide a mechanism for independent verification. It's a starting point, but not a tool for verification.
B. Internal audit: Internal audits are valuable for internal control and improvement, but they lack the independence required to verify CSP claims to external customers. There may be bias in the audit result.
D. Security assessment: Security assessments typically focus on identifying vulnerabilities and risks, but they don't necessarily verify compliance with the shared responsibility model as broadly and objectively as an external audit does.

The Cloud Security Alliance (CSA) provides guidance on cloud security and governance, emphasizing the importance of independent assessments and audits.

The shared responsibility model dictates that cloud providers are responsible for the security of the cloud, while customers are responsible for security in the cloud. To ensure CSP's claims are valid, external auditing is necessary.

The AICPA SOC 2 reports are examples of external audits performed by certified public accountants. These reports can be used to verify CSP claims.

Selecting an appropriate audit will allow for a comprehensive review of security and compliance controls.

Customers benefit from the assurance provided by independent reviews and validation of CSP claims.

An external audit offers an objective and independent evaluation of the CSP's adherence to the shared responsibility model, providing customers with the necessary confidence in the CSP's claims.