Question

If an adjective ends in an "o," you change it to an ______ before adding the adverbial ending.

e

a

Answer 1

here are some example from which you can see the rule:

completo/complatemente
exito/existosamente
enojado/enojadamente

from that you should be able to figure out the answer

Answer 2

Just for future reference the answer is it ends in "a". I just took the test and got it right. I'm sure @TuringTest was not trying to mislead.