The correct answer is: B. didn’t see
The reason is that the sentence is in the past tense, and the verb “see” is irregular. The past tense of “see” is “saw,” but the negative past tense is “didn’t see.” This means that the speaker did not see much of the film because they fell asleep half-way through.
The other options are incorrect because they do not make sense in the context of the sentence. Option A, “had seen,” is in the past perfect tense, which is used to talk about something that happened before another event in the past. Option C, “wasn’t seen,” is in the passive voice, which is not used in this context. Option D, “saw,” is the simple past tense, which is not correct because the sentence is in the past tense.
In conclusion, the correct answer is B. didn’t see.