Final answer:
Using Task Manager to end the unresponsive task is the best action for the user.
Step-by-step explanation:
If a user working on a spreadsheet faces an unresponsive application, the best action for the user to take is to use Task Manager to end the task. Task Manager is a utility in Windows that allows users to monitor and control running processes, and can be used to forcefully terminate unresponsive applications. Continuing to wait may not resolve the issue, and repairing or reinstalling the application should be considered as a last resort if Task Manager does not solve the problem.