Option 1 -> "will have started" is future perfect tense, but "reached" is past tense, so this doesn't match.
Option 2 -> "had started" is past perfect tense, correctly showing the movie started before we reached the theatre.
Option 3 -> "has started" is present perfect tense, which doesn't align with the past tense "reached."
Option 4 -> "is starting" is present continuous tense, incompatible with the past context.
Hence, Option 2: had started -> When two actions occurred in the past, we use past perfect tense for the action that happened first. Since the movie started before we reached the theatre, "had started" is the appropriate choice to show this sequence of past events -> correct