En la escuela un gimnasio y un teatro tambien. a. comía b. era c. había

Explanation:

This is a sentence written in imperfect tense, which is used to talk about past habitual actions or to describe what someone was doing when they were interrupted by something else.

In other words:

It is specially used to describe what used to happen or what was happening.

Here we are talking about a school where there was a gym and a theater.  

In Spanish if we want to express that someone or something exists, we use the verb haber (to have) also equivalent to the phrase there (is/are/was/were).

Now, the imperfect tense of the verb haber with the 3rd person in singular is había. Therefore the complete sentence is:

In the school there was a gymnasium and a theater too

The options comía (used to eat) and era (was/were), although are also in imperfect tense, do not match with the context

En la escuela había un gimnasio y un teatro también.

Explanation:

The option “había” is correct because the sentence is expressing the presence of two areas in the school. The verb “había” is the past tense of the verb “haber” (conjugated in third person for this case) which denotes the presence or possession of something.

The other two options are also conjugated in past tense and can be used when talking about a third person. However, the verbs themselves express totally different actions. “Comía” refers to the verb “comer” (to eat), whilst “era” refers to the verb “ser” (to be).