I believe it would be Sera since we're talking about Carlos as a "him".
The ones with la are feminine and the ones with el are masculine. When the noun ends in an “a” it’s obviously feminine and when a noun ends in “el” it’s obviously masculine. It’s hard to tell whether it’s masculine or feminine when theres los and las and when it doesn’t end in an “a” or “e”
Answer:
¿Por que estudias español?
Why are you learning Spanish?
Sarah practices on her bike so she can win the race ...
Spanish: Hay dos butacas pequeñas en la sala familiar.
English: There are two armchairs in the family room.
I hope this helps you.