The second option is correct, because in Spanish, its "Voy a beber agua.", but in English, its "I am going to drink water." The other options do not make any sense in this question.
All in English:
What will you drink?
You are going to drink water. This answer is wrong.
I am going to drink water. This answer is right.
We are going to drink water. This answer is in second person, not first person, so wrong answer.
You will drink water. This is in third person, the person is asking you, so this answer is totally wrong.
Hope this helped!
Nate