The correct answer is who.
Which is used to refer to things, not people. In this sentence, the girl is a person, so we cannot use “which”.
Whom is used to refer to the object of a verb or preposition. In this sentence, the girl is the subject of the verb “broke”, so we cannot use “whom”.
Whose is used to show possession. In this sentence, we are not talking about the girl’s possession, so we cannot use “whose”.
Therefore, the correct answer is who.
Here is a more detailed explanation of each option:
Which is used to refer to things, not people. For example, we might say “I like the car which is red”. In this sentence, the car is a thing, so we can use “which”. However, in the sentence “The girl, who broke the mirror, was scolded by her mother”, the girl is a person, so we cannot use “which”.
Whom is used to refer to the object of a verb or preposition. For example, we might say “I gave the book to whom?”. In this sentence, “whom” is the object of the verb “gave”. However, in the sentence “The girl, who broke the mirror, was scolded by her mother”, the girl is the subject of the verb “broke”, so we cannot use “whom”.
Whose is used to show possession. For example, we might say “The girl whose car is red is my friend”. In this sentence, “whose” shows that the car belongs to the girl. However, in the sentence “The girl, who broke the mirror, was scolded by her mother”, we are not talking about the girl’s possession, so we cannot use “whose”.