To determine the correct verb form to fill in the blank, let's analyze the sentence structure and the tense of the verb.
1. Sentence structure:
The sentence is in the past tense because the word "before" indicates that the action happened in the past. Since the sentence is in the past tense, we need to use a past participle form of the verb.
2. Tense of the verb:
The verb "see" is an irregular verb, and its past participle form is "seen." Therefore, we need to use the past participle form of the verb "see" in the sentence.
3. Options analysis:
- Option A: "seen" - This is the past participle form of the verb "see," but it is not the correct form because it lacks the auxiliary verb needed for past perfect tense.
- Option B: "had seen" - This is the correct answer. It uses the past perfect tense, which is formed by combining the auxiliary verb "had" with the past participle form of the verb "see."
- Option C: "have seen" - This is the present perfect tense, which is incorrect because the sentence is in the past tense.
- Option D: "has see" - This is incorrect because it uses the present tense instead of the past tense.
Therefore, the correct answer is option B: had seen.