In an agile framework, it is beneficial to review the completion costs of similar projects as it provides a realistic baseline for the new project's cost estimation. This approach is supported by the PMBOK guide, which suggests using historical information and lessons learned from similar projects to inform cost estimations1. Additionally, involving the client representative (option A) is also a good practice in agile projects to ensure that the estimation aligns with the client's expectations and priorities2. However, the primary action should be to review similar past projects for a more accurate estimation.
References: The PMBOKGuide outlines the importance of utilizing historical data and information from similar projects when estimating costs3. The PMP Examination Content Outline also emphasizes the use of past project data for cost estimation in agile project environments1. The Professional in Business Analysis Reference Materials further support this approach, highlighting the value of lessons learned and historical analysis in project cost management4.