1.Everyone is wearing (his or her, their) costume tonight.

2.Neither of the actresses wore (her, their) hat.
3.Did anyone forget to bring (his or her, their) notes.
3.I don't know if either of the girls will sing (her, their) lines tonight.
4.Somebody left (his or her, their) uniform behind stage.
5.Everyone sold (his or her, their) tickets last week.

1.his or her
2.her
3.her
4.his or her
5.his or her

All are correct -- except you have two different sentences for # 3.

Oops I guess I numbered them wrong.

1.his or her
2.her
3.his or her
3.her
4.his or her
5.his or her

Right. Thanks for clarifying.

Thank you.

Nope

Hi I'm good

The correct pronouns to use in these sentences are as follows:

1. Everyone is wearing their costume tonight. (Use "their" as a gender-neutral pronoun to refer to a group of people)
To determine the correct pronoun in this sentence, you can consider the subject "everyone." Since "everyone" refers to a group of people, the pronoun "their" is used instead of "his or her."

2. Neither of the actresses wore her hat. (Use "her" for a singular female noun)
In this sentence, "neither" refers to a singular noun (actress), so the pronoun "her" is used.

3. Did anyone forget to bring their notes? (Use "their" as a gender-neutral pronoun)
Similar to sentence 1, "anyone" refers to an unspecified group of people, so the pronoun "their" is used.

4. I don't know if either of the girls will sing her lines tonight. (Use "her" for a singular female noun)
In this sentence, "either" refers to a singular noun (girl), so the pronoun "her" is used.

5. Somebody left their uniform backstage. (Use "their" as a gender-neutral pronoun)
Again, "somebody" refers to an unspecified person, so the pronoun "their" is used.

Remember, the use of pronouns can depend on the context and personal preference.