Where should a fuse or circuit breaker be installed in a 120V AC power circuit?

Answer: A. In series with the hot conductor only

A fuse or circuit breaker goes in series with the hot conductor only. In series, all the current passes through it, so it can cut the power. Putting it in the neutral could leave equipment still connected to the hot wire after the fuse opens.

Tip: Fuse in series, on the hot wire.