Which of the following tools would BEST diagnose the possible bottleneck?

A PC is running slower than usual. Which of the following tools would BEST diagnose the possible
bottleneck?

A.
CHKDSK

B.
Device Manager

C.
Task Scheduler

D.
Task Manager

Explanation:
http://www.ictc.com/my-computer-is-running-slow-what-can-i-do/