Right answer:
D. vendiéndome
The word vendiéndome includes the indirect object pronoun for the first person singular, which is me. An Indirect Object Pronoun is a person that receives the action of a verb indirectly. Its purpose is to tell you to whom or for whom something is done. So this action is being performed for me. This word comes from the verb vender but here it is being used as gerund (the verb form that ends in -ndo). In fact, an indirect object pronoun can be written at the end of the verb if and only if the verb is not conjugated, such as infinitives or gerunds or if the verb is an affirmative informal command. Finally:
Lupe: El vendedor está vendiéndome el sofá y le doy dinero al vendedor.