The correct phrase to complete the sentence "No me mustang los panalones negros; prefiero ?" is "Los Blanco" (option 3).
So the complete sentence would read: "No me mustang los panalones negros; prefiero los Blanco."
However, it seems there may be a typographical error or confusion in the phrasing "mustang" which should probably be "gustan" (which means "like" in Spanish). The corrected sentence would be: "No me gustan los pantalones negros; prefiero los Blanco."