Object Pronoun or possessive pronoun

I wonder why option (C) is wrong? I think it is still possible that the possessive pronoun “hers” is used to replace “her birthday”

Ms. Greelay’s colleagues, who discovered that her birthday is on Thursday, are preparing a special celebration for ----------- (A) her (B) she (C) hers (D) herself

If the pronoun is supposed to mean "her birthday," then C is the only right answer. It's the only one that can mean "her birthday."

If the right answer is A, then it doesn't mean "her birthday" but means "Ms. Greelay."

Anyway, you might hear that sentence with "hers" in a context of:

Mr. Johnson's colleagues know it's his birthday on Thursday and aren't doing anything special for his birthday. Ms. Greelay’s colleagues, who discovered that her birthday is on Thursday, are preparing a special celebration for hers.