The correct word to complete the sentence is "où."
The full sentence would be: "Où habite ton ami?" which translates to "Where does your friend live?"
However, if you are specifically looking to complete the sentence with one of the options you provided, please provide more context for me to assist you accurately. The words "vas," "va," "allez," and "habite" relate to different subjects and conjugations in French, but without additional context, I can't select the appropriate one among them.