During project execution, a project manager discovers that the budget at completion has shifted significantly...

 According to the PMBOK Guide, a root cause analysis is a technique used to identify the underlying reasons for the occurrence of a problem or a deviation from the expected performance. It is part of the monitor and control project work process, which involves tracking, reviewing, and reporting the project progress and performance, and implementing corrective or preventive actions as needed. A root cause analysis can help the project manager to understand the causes of the budget shift and to take appropriate actions to address them. It can also help to prevent similar problems from happening again in the future. Adjusting the original budget estimates, negotiating the project changes, or reviewing the activity duration are possible actions that the project manager can take after performing a root cause analysis, but they are not the first step to do when faced with a significant budget shift. References:

PMBOK Guide, 6th edition, pages 121-122, 4.5 Monitor and Control Project Work

PMBOK Guide, 6th edition, pages 686-687, Root Cause Analysis

PMP Sample Test Questions, question 1, answer D