The sentence has one event occurring i.e finishing lunch. So we cannot use the past perfect tense (this would require two events) which rules out options A and B. The event happened half an hour ago. So, it is in the simple past tense. Option D) have finished is in the present perfect tense so it is incorrect. Therefore, the best answer is option C) finished. Also, an important thing to notice is the adverb of past time (an hour ago) and when that is mentioned in the sentence, the verb is in the simple past tense.