When the project manager identifies that the delay on the critical path was caused by the client, it is essential to formally document this and then seek a discussion with the client. This supports transparency, allows for resolution, and protects the contractor from inappropriate penalties.
Per the PMBOK Guide - Seventh Edition and PMI's Contract Management standards, documenting the root cause and opening a dialogue is the first proper step in addressing contractual disputes.
Option B is a valid legal step, but only after engagement and documentation.
Option C refers to an internal party (sponsor) who may not be relevant here.
Option D may support analysis but does not immediately address the contractual issue.
References:
PMBOK Guide - Seventh Edition, Governance and Delivery Performance Domains PMI Practice Standard for Project Contracts