The correct response to complete the question "___ du gern Freunde?" is "Bist".
So the complete sentence would be: "Bist du gern Freunde?"
However, please note that the phrase is a bit awkward in German. A more natural way to ask would be "Hast du gern Freunde?" or "Möchtest du Freunde?" which translates to "Do you like having friends?". But from the responses given, "Bist" fits the grammatical structure.