The correct answer is D. were sleeping.
The simple past tense is used to talk about events that happened at a specific time in the past. The past participle is used to form the perfect tenses, which are used to talk about events that happened before another event in the past. The past perfect continuous tense is used to talk about events that were happening at a specific time in the past.
In this sentence, the accident occurred when the passengers were sleeping. This means that the accident happened while the passengers were in the middle of sleeping. The past perfect continuous tense is the most accurate tense to use in this situation because it emphasizes the fact that the passengers were sleeping for a period of time before the accident happened.
The other options are not correct because they do not accurately describe the time frame of the accident. Option A, sleep, is in the simple present tense, which is used to talk about events that happen regularly or repeatedly. Option B, had slept, is in the past perfect tense, which is used to talk about events that happened before another event in the past. Option C, had been sleeping, is in the past perfect continuous tense, which is used to talk about events that were happening at a specific time in the past.
In conclusion, the correct answer is D. were sleeping.