Answer:
A wife
Step-by-step explanation:
After Faustus decides to sell his soul, he asks Mephastophilis for a wife. However, instead of offering a human wife, Mephastophilis offers him a female demon as a servant. Faustus dislikes this idea, and he refuses to accept. Mephastophilis gives him a book of magic spells instead, and asks him to study it carefully.