Final answer:
The lesson from the Grandma story for project managers is that sunk cost should not influence future project investments.
Step-by-step explanation:
The subject of this question is Business. The question is related to the lesson on sunk costs and its application for project managers.
The correct option for the lesson from the Grandma story for project managers is C. Sunk cost should not influence future project investments because it can lead to throwing more resources into a failing course of action. This lesson highlights the importance of making decisions based on future outcomes rather than being influenced by past investments.
Ignoring sunk costs allows project managers to objectively evaluate the potential benefits and risks of a project and make informed decisions about continuing or discontinuing it. It prevents them from falling into the trap of pouring more resources into a failing project just because they have already invested a significant amount of time, money, or effort in it.