Final answer:
The Task Manager is the most suitable tool to diagnose a PC that is running slower than usual as it shows resources usage and which processes are consuming them.
Step-by-step explanation:
To diagnose a PC running slower than usual, the Task Manager would be the best tool to use. The Task Manager provides an overview of the system's resources such as CPU, memory, disk usage, and network activity. It can show which processes and applications are consuming the most resources, thus helping to identify possible bottlenecks in the system.
Options like CHKDSK are used for checking disk integrity and can fix various file system errors, but it's not primarily used to diagnose performance issues. The Device Manager is helpful for checking hardware and driver issues, while the Task Scheduler manages scheduled tasks and isn't directly related to monitoring system performance.