Answer:
estás
Explanation:
From this point of view, Marco is asking a person directly, so he would use estas, instead of any other form of conjugation.
Secondly, eres is used to describe a permanent trait about a person; the way that Luisa is feeling, is only temporary, therefore you should not use Eres.
Cree en es the correct answer
El cree que puede correr más rápido
