Answer: C (est)
Explanation:
Here's a list of all the forms of être:
je suis (I am)
tu es (you are)
il/elle est (he/she/it is)
nous sommes (we are)
vous êtes (you are)
ils/elles sont (they are)
Hope this helped!
(P.S. don't think this should be in Mathematics, I think this is French)
(as in the subject)