"Whose milk are you drinking?"
Translation:Чьё молоко ты пьёшь?
Both in a way. It needs to agree with its noun - in this case молоко - in gender and case. Молоко happens to be neuter, and in this sentence it's in accusative because the verb пить "to drink" wants an object in accusative case. So "whose" has to appear in the neuter accusative form as well to match молоко (although luckily the accusative form is always identical to the nominative for neuter (and singular) nouns).