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

Tobias wolff most likely named his short story "mortals" because the story explores the theme of people coming to terms with the idea that one day they too will die. wanting to find out what their loved ones really think of them. achieving immortalit by being "big people" while they are alive. hoping that they will have accomplished something before they die.