The correct answer is: A. was sitting
The sentence is in the past tense, so the verb "to sit" should also be in the past tense. The other options are incorrect because they are not in the past tense.
- B. sat is also in the past tense, but it is the simple past tense, which is used to talk about a single action that happened at a specific time in the past. In this sentence, the speaker is talking about a continuous action that happened over a period of time, so the simple past tense is not the correct tense to use.
- C. have been sitting is in the present perfect continuous tense, which is used to talk about an action that started in the past and continues up to the present. In this sentence, the speaker is talking about an action that happened five years ago, so the present perfect continuous tense is not the correct tense to use.
- D. No improvement is not an option, but it is worth mentioning that this sentence is grammatically correct. However, it is not the most idiomatic way to say this. A more idiomatic way to say this would be: "Five years ago today, I was sitting in a small Japanese car, driving across Poland towards Berlin."