"What do his legs want?"
Translation:उसके पैरों को क्या चाहिए?
"को" acts here as a marker of the dative/indirect object ("to, for X"). It's used, among other things, for someone who receives something given ("I give HIM the gift"), or who experiences something ("To ME, there is cold" -> "I am/feel cold"). In Hindi, the normal way of describing wanting is to say that something "is wanted to/for" someone ("To his legs, what is wanted?" -> "What do his legs want?").