Answer for yourself, but make the preference say something. ‘Vocal’ or ‘instrumental’ on its own is not an answer; the reason is the answer.
Sample answer: I prefer instrumental music, especially while studying. Vocal music brings words with it, and words demand attention — I find myself following the story of a song instead of the page in front of me. An instrumental piece gives me the same mood without any language to compete with my own thoughts. That said, I switch to vocal music the moment I stop working: a song can name a feeling exactly, and no flute solo has ever made my grandmother sing along the way a Bihu number does. So the honest answer is that instrumental music is for concentration and vocal music is for company.