The answer you're looking for is Her.
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The reason behind is this is because for pronouns you'll go off of the person/people's gender. When reading the provided sentence you'll see that we learn that the person who forgot their part is female. How do we know this? Well, we know this because the person is in a "girls" choir which is specifically made for females. Now, we need to know which pronoun to use. Using basic knowledge, we know that she/her are pronouns used for those whom are female. So, the correct answer will be "her."
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If you have an questions related to this you can go off this quick little chart I wrote for you!
Female (or girl) - She/Her
Male (or boy) - He/Him
More than one person - They/Them
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Good luck!
- FantasticFerret<3