What spanish pronoun do I use here?

[Amy C]

I don't know which spanish pronoun to use in the phrase "your (informal, plural) cat."

The choices for the answer are:
tu gato
tus gato
su gato
sus gato

My class does not use vosotros.

My teacher says the correct answer is "su gato," but I do not understand why I would use "su" because she already told us usted/ustedes is formal, instead of informal like the question says. If she is correct, please explain why.

Thank you!
The cat in question is singular.
I know that tu is informal. However, tu is singular, or so I thought.

Ustedes is plural, but it is formal.

That is why I am confused.
The people being addressed by the pronoun are plural, not the cat.