A user's track pad has stopped working.

The correct answer is B. Use the Function key.

The explanation is that some laptops have a Function key (Fn) that can be used to toggle the track pad on or off. This can be useful if the user wants to use an external mouse instead of the track pad, or if the track pad is accidentally activated by palm movements. The user should look for a Fn key on their keyboard and a track pad icon on one of the other keys, such as F5 or F9, and press them together to enable or disable the track pad.