Correct answer:
comía
In Spanish, the imperfect tense is used to describe past habitual actions or to talk about what someone was doing when they were interrupted by something else. In general, the imperfect is used to talk about past actions, conditions, or events that occurred regularly or frequently or that were in progress in a moment in the past. Since this sentence stands for the third person singular, to conjugate the verb comer take away the er ending and add ía to the end of the word. Finally:
Marta comía en un restaurante.