Match the part of speech (in bold) to their usage(down)

In sentence 5, "down" indicates the relation of the fire engine with the hill. Thus, it is used as a "preposition". Hence, 2-5 is a match.

Options C and D can be eliminated.

In sentence 8, "down" is used to describe a noun [train]. Thus, it is used as an adjective. Hence, 1-8 is a match.

Hence, option A is the answer.

In sentence 6, "ups" and "downs" are nouns. Hence, 3-6 is a match.

In sentence 7, "down" denotes action and is thus, used as a verb. Hence, 4-7 is a match.