The correct response to the question: ¿A quién se lo das? Is:
c. Se lo doy a Marta.
Explanation:
You can see the structure form of the question: ¡A quién selo das? Te underlined words are object pronouns that must be used in the answer, in the case of the verb, the question refers to you, by that reason the answer must be with the personal pronoun I or in Spanish"yo" and the correct conjugated form of the verb "dar" for that noun is "doy".
