On a road with two lanes traveling in opposite directions, slow-moving vehicles should pull over when safe to let others pass when:
A. Two or more vehicles form a line behind you and it is unsafe to pass.
B. Five or more vehicles form a line behind you and it is unsafe to pass.
C. The road is posted with slow-moving vehicle signs.
D. One vehicle is behind the slow-moving vehicle.

D. One vehicle is behind the slow-moving vehicle.

Explanation:

there are no options provided but you can choose among the following gender-neutral pronouns: zir, hir, eir, vis, tem, eir.

in the normal course of maintaining a pronoun-antecedent agreement, we could have used the words his or her but since we are also required to use gender-neutral language, we need to replace it with its equivalent gender-neutral pronoun.