D. When Faustus is in the grove performing an incantation.
Explanation:
Mephistopheles initially shows up at Faustus' solicitation and discloses to him that he must be his servant if Lucifer consents to it.
Mephistopheles re-shows up, conveying word that Lucifer has consented to Faustus' proposition if Faustus will sign his spirit over to him in a deed of blood.
With the bloody contract signed, Mephistopheles shows Faustus the idea of hellfire and the devil, and about astronomy.
