¿quién se comió mi comida? melanie se comí tu comida. melanie se comieron tu comida. melanie se comió tu comida. melanie se coma tu comida.

Firstly the question is made in Simple Past Tense with the third person in singular (because the answer to WHO is she, he or it):

¿Quién se comió mi comida?

Who ate my food?

In spanish (castillian) gramar the Simple Past Tense is used to express specific actions completed or concluded in the past that are separated from the present.  

In other words:

Therefore, the answer must be in concordance with the question in verbal tense and the subject pronoun:

Melanie (she) ate your food

Additionally, note the usage of the reflexive pronoun se, which in this case only intensifies the meaning of the verb comer (to eat), this is called the reflexive dative pronoun in spanish. It is when the subject and the complement of the verb are the same.

This is why the right answer is Melanie se comió tu comida