The correct answer is D because the scenario requires disciplined schedule management rather than an immediate, informal, or purely administrative reaction. The selected response - D. Refer the department manager to the schedule management plan. - directly addresses the condition described in the question and keeps the project aligned with authorized objectives. PMBOK-aligned practice requires the project manager to analyze dependencies, determine schedule impact, and select a corrective action that protects the critical objectives. This is especially important when project constraints, stakeholder expectations, quality expectations, delivery cadence, or business value may be affected. The competing options are weaker because they either bypass the accountable role, defer action without analysis, escalate before the project manager has prepared decision-quality information, or solve a symptom without correcting the underlying process issue.
The selected answer also preserves transparency, ownership, and traceability, which are essential for sound project governance and effective delivery. PMBOK Guide Eighth Edition topics: process, schedule management, stakeholder communication, impact analysis, governance, value delivery, and continuous improvement.