How is the underlined pronoun used in the sentence?

((who)) at the park left a pair of mittens on the ground?

a.
subject

b.
direct object

c.
possessive

d.
object of a preposition

A subject is the one being discussed in the sentence. In the question about, Who functions as the subject. It is the receiver of the action-- left a pair of mittens on the ground. The direct object is the pair of mittens. It's not possessive because it's not asking about ownership. Park is the object of the preposition.
ANSWER: A