Bonjour
What are you doing ?
CORRECT ANSWER = B ..................
Qu'est-ce que tu fais ?
A- Que fais-tu ? is quite similar = What do you do ? (it's just more formal)
C- Comment tu fais ? = How do you do ? = FALSE = comment ?= how ?
D-Tu fais quoi ? = is a way to talk, same meaning, but grammatically incorrect.............
when you ask a question with "qu'est-ce que .." it usually means 'at the moment' .. and it's unformal ...
the real difference between "Qu'est-ce que ..?" and " "Que fais-tu? " is that the first one is informal, the second one formal ...........
So I don't really know what your teachers are teaching you about that