Okay, an antecedent of a pronoun is always a noun, so immediately you can reject the words late and wandered as incorrect answers, because late is an adjective, and wandered is a verb. This leaves us with two nouns, guests and party. Obviously, the pronoun several refers to the noun guests, so the correct answer is guests - that is the antecedent that corresponds with the pronoun. Several guests, not several parties.
The correct answer is A.
Gregor's mother has been prevented from seeing Gregor for some time. When she finally sees him on the wall, covering the photo of the woman in furs, she falls onto the sofa and faints from fear. She is terrified of his new appearance.