what is never to be engaged when using power tools?\ntrigger lock\nbit shank\nbooster

Brief Explanations:

The trigger - lock is used to prevent accidental activation of power tools and should be engaged when not in use. A bit shank is a part of a tool bit that fits into the tool's chuck. A booster is not a standard feature that should never be engaged. The trigger - lock is something that should be engaged for safety, not left un - engaged.

Answer:

C. booster