In this situation, the best course of action for the Project Manager (PM) is Option C: Meet with the key stakeholders to understand why they feel the agile method being used is not efficient enough.
Here’s why this is the recommended approach:

1. Open Communication: Before making any decisions or taking drastic actions, it’s crucial to have an open and transparent discussion with the key stakeholders who have raised concerns. This demonstrates a willingness to listen and understand their perspective, which can help address their concerns more effectively.
2. Gather Information: By meeting with the stakeholders, the PM can gather valuable insights and feedback about their specific issues with the current delivery schedule and agile approach. This information will help the PM identify the root causes of the stakeholders’ dissatisfaction and make more informed decisions.
3. Build Trust and Alignment: Engaging in a dialogue with stakeholders fosters trust and collaboration. It shows that their concerns are being taken seriously, and it provides an opportunity to align expectations and address any misunderstandings or misconceptions about the agile methodology.
4. Explore Solutions: During the meeting, the PM and stakeholders can explore potential solutions or adjustments that could address the stakeholders’ concerns without necessarily abandoning the agile approach altogether. This collaborative approach increases the chances of finding a mutually agreeable solution.

The other options provided have their drawbacks:
Option A (Change the agile method to a predictive method) is a drastic measure that should not be taken without first understanding the stakeholders’ concerns and exploring alternative solutions within the agile framework.
Option B (Meet with the team to understand why the delivery schedule is being delayed) is important, but it should be done after gathering input from the stakeholders, as their concerns might not be solely related to the delivery schedule.
Option D (Inform the key stakeholders that this is the best way to deliver a product) dismisses the stakeholders’ concerns without engaging in a dialogue, which could further erode trust and collaboration.