When key team members resign, it's crucial to address the situation promptly to minimize the impact on the project. Option C, acquiring replacements by initiating the hiring process immediately, is the most proactive approach. It allows the project manager to fill the vacant positions with qualified individuals as quickly as possible, helping to maintain the project schedule.
Option D, escalating to the project sponsor for a schedule delay, should be considered only if acquiring replacements is not feasible within the current timeline. However, the preference is generally to address resource gaps promptly to avoid project delays.