Rohan had watched the movie before he read the review. This means that Rohan watched the movie in the past, and the action of watching the movie happened before the action of reading the review. In English grammar, this is known as the past perfect tense.

Explanation:
- The past perfect tense is used to indicate that an action happened before another action in the past. In this case, the action of watching the movie happened before the action of reading the review.
- The past perfect tense is formed by using the auxiliary verb "had" followed by the past participle of the main verb. In this case, the main verb is "watched," and its past participle form is also "watched."
- The sentence "Rohan had watched the movie before he read the review" follows the correct structure for the past perfect tense: subject + had + past participle + before + subject + past simple.
- The past simple tense is used for the action that happened after the action in the past perfect tense. In this case, the action of reading the review is described in the past simple tense.
- The other options given in the question are not correct because they do not accurately describe the sequence of events. Option 'a' ("watches") is in the present simple tense, option 'b' ("have watched") is in the present perfect tense, and option 'd' ("was watching") is in the past continuous tense. None of these options indicate that Rohan watched the movie before he read the review.

In conclusion, the correct answer is option 'C' ("had watched") because it accurately describes the sequence of events, with Rohan watching the movie in the past before reading the review.