During execution, the project manager finds the budget at completion has increased significantly above expectations. What should they do?

A significant increase in the Budget at Completion (BAC) indicates a major deviation from the plan, requiring a thorough investigation to understand why it happened. A root cause analysis (RCA) identifies the underlying reasons for the variance, allowing the project manager to implement effective corrective actions and prevent recurrence. Adjusting original budget estimates without understanding the cause is reactive and doesn't address the problem. Reviewing activity durations focuses on schedule, not the core budget issue. Negotiating changes and resetting expectations might be necessary after understanding the root cause, but it's not the immediate first step to address a budget overrun.