A number of unexpected issues have occurred during the execution of a project that will...

Because the issues are already occurring and are expected to negatively impact schedule and cost, the most direct mitigation action is to use contingency reserves/buffers that were planned for uncertainty. CompTIA Project+ explicitly includes both determining contingency reserves/buffers during planning and contingency reserve/buffer utilization during execution/controls as part of schedule and project control activities.

Updating the issue log is important for tracking, ownership, escalation, and outcomes, but it is primarily a documentation/control step, not the immediate mitigation that protects the timeline and budget. Communicating with stakeholders is also essential (especially if reserves will affect commitments), but communication alone doesn’t reduce the impact—reserves do. Reviewing the risk register can help connect issues back to known risks and validate whether contingency plans exist, but the question asks what is best to do to mitigate the schedule/cost impact now; that is exactly what contingency reserves are designed for.

In practice, the PM would typically: document the issues, apply the reserve/buffer, update schedule/budget baselines as needed, and then communicate the revised forecast—however, the mitigation action among the options is using contingency reserves.