Question

On what assumption was the encomienda system based?

that Europeans had the right to demand labor from Native Americans
that exposure to European culture would help educate Native Americans
that living on Spanish plantations would convert Native Americans to Christianity
that mestizos had more rights than full-blooded Native Americans

Answer 1

Not the mestizos one

Answer 2

And not right to demand labor

Answer 3

B and C are close to the same idea. Queen Isabella didn't allow slavery but the encomienda system was practically slavery.. A big part of the encomdios responsibility was to make sure natives learned Spanish and Christianity. So I'm going with C