What circuit that has only one path for the
current to follow?

Series circuit

Explanation:

By definition series circuit is a closed circuit in which the flow of electric current follows only one path.

In a simple electrical circuit (which is a series circuit) where electric cell is the source of power, each component is connected in such a way that there is only one pathway current flows from the positive terminal(+) round to the negative terminal(-) via the external circuit.

Current having only one pathway to flow means that when one of the components is removed, there will be no current flow.

t = 1/400 h   ⇔ 0.0025 h ⇔ 9s ⇔ 0.15 m

any of the answers are correct.