आपका means your (polite form), while अपना is one's own, that is when the subject (in this case you) is the possessor of the object. In this sentence only अपना is correct. If it was for example I drink your water or He drinks your water, then it would be आपका.
I think that "You are drinking your water" should also be added to the list of possible correct answers.
Is this being told to a boy or a girl? Apna makes me think that a boy is being told, but piti makes me think that it is a girl...this is confusing.