The correct answer is: A. has
The car has been leaking oil. This is because the present perfect tense is used to talk about actions that started in the past and continue to the present. In this case, the car has been leaking oil for some time, and it is still leaking oil.
The other options are incorrect because they do not accurately describe the situation. The past tense (B. had) would be used to talk about an action that happened in the past and is no longer happening. The past participle (C. were) would be used to form the perfect passive voice, which is not appropriate in this case. The past participle (D. was) would be used to form the simple past tense, which is also not appropriate in this case.
To summarize, the correct answer is: A. has. The car has been leaking oil. This is because the present perfect tense is used to talk about actions that started in the past and continue to the present.