"O ne içer?"

Translation: What does he drink?

[AbdelhalimAH]

Is "What is he/she drinking?" also a correct answer?

[scarpunzel]

I don't think so, because -ing is continuing present. I believe that is handled differently Turkish.

[sebyte]

"What does she drink?" should be a correct answer (as well as 'he').

[JasoonS]

Is 'O içer ne' also acceptable?

[AlexinNotTurkey]

No....question words will never ever come after the verb. It must receive stress and will always occur before the verb for that reason :)