One project phase took much longer than planned and key stakeholders are worried about a possible delay. What should the project manager do?

Organizing a root cause analysis (RCA) workshop is the most appropriate action. This allows the project manager to identify the underlying reasons for the delay, rather than just addressing the symptoms. Understanding why the phase took longer enables the team to develop effective corrective actions, prevent recurrence, and make informed decisions about future phases. Requesting additional contingency from the sponsor is premature without understanding the cause; it's a reactive measure that doesn't solve the core problem. Adding more float to the overall project schedule is not a proactive solution and might mask underlying issues without addressing them. Expediting the next phase to reduce slippage could lead to quality issues, increased costs, or burnout if the root cause of the previous delay isn't understood and mitigated.