"What will you drink?"
Translation:तू क्या पियेगा?
By 'Duo' you mean 'the volunteer contributors' - I see you do pay for 'plus', so you're not complaining about something you're getting for free like most people, but do bear in mind that that cash doesn't go toward the people that give up some of their time to offer the course and review reported errors.
(I'm not one of them and bitter about it, just saying!)
I keep trying तुम क्या पियोगे, keep getting that as wrong, keep reading the comments and I keep reporting this question almost on a weekly basis, and yet the question keeps ruining my streak each and every time :'D Maybe I should use the आप form as standard instead of तुम in these exercises.
Yes. This form of 'you' is reserved for very close friends and family of a similar age as yourself and using it for anyone else would be considered rude. Moreover, in certain regions, people steer clear of तू altogether.
As a Hindi learner, it is best that you use आप as the default form for someone you've just met and switch to तुम once you are comfortable with that person.
पिऊँगा is only used when the subject is मैं (I).
पिएगा is used when the subject is तू (informal form of 'you') and with third-person singular subjects.
If you are getting confused with the future-tense forms, you can have a look at my comment in this discussion. I have listed all possible future tense forms for three verbs.