Options A and C: 'What' is a question word that asks about a specific thing.
'Which' indicates a choice between things or people.
The given sentence asks for a choice between students. Hence 'which' is preferred over 'what', and option C is correct; A is incorrect.
Option B: 'How' refers to the manner in which something is done. It doesn't apply here, hence B is incorrect.
Option D: 'Who' refers to person; but it does not refer to choice. Hence this option is incorrect.