"We got to the hall after the play ...."

The verb "got" is past tense, so we are looking for a verb that shows an action that happened before that point in the past.

That means we need a past perfect tense to describe the earlier action: "had started."

Option analysis:
A. is started – Present tense/passive; grammatically wrong here.
Incorrect

B. was starting – Past continuous, but doesn't clearly show that it was already underway before they arrived.
Incorrect

C. had started – Past perfect, showing that the play started before they arrived.
Correct

D. has started – Present perfect; not suitable with "got" (past tense).
Incorrect

E. would start – Future in the past; doesn't fit the sequence.
Incorrect