The correct answer is B - The customer changed requirements. Agile teams embrace change, especially when it improves the product's value. Customer-driven changes to requirements are the most common and legitimate reason for adjusting project plans in agile, supported by rolling wave planning and continuous backlog refinement.
From the PMI Agile Practice Guide:
"Agile projects anticipate changes in requirements. The product backlog is continuously updated to reflect new insights and customer needs." (PMI Agile Practice Guide, Section 5.3 - Rolling Wave Planning) Mike Griffiths emphasizes:
"In agile, planning is dynamic. The most common source of change is evolving customer requirements, which lead to changes in scope and prioritization." (Mike Griffiths, PMI-ACP Exam Prep, Chapter 5 - Adaptive Planning) Incorrect options:
* A is unrelated and unrealistic.
* C is useful but doesn't directly alter the project plan.
* D is ambiguous and doesn't reflect business-driven change.