Correct Answer & Explanation
Option C
• This sentence describes two completed actions in the past: 'reaching the theatre' and 'the movie starting'.
• When two actions occur in the past, the action that happened first is expressed in the Past Perfect Tense, and the later action is in the Past Indefinite Tense.
• The movie started *before* I reached the theatre.
• The structure for Past Perfect Tense is Subject + had + V3 + Object.
• Therefore, 'had started' correctly indicates the earlier action.