The missing word is hadn't sold.

The sentence is saying that the speaker regrets selling their car last week. The speaker wishes that they had not sold their car. The correct way to express this sentiment is to use the past perfect tense of the verb sell, which is had sold.

The sentence would then read:

I sold my car last week. It's my biggest regret. I wish I hadn't sold my car.

The missing word is kept.

The correct sentence is:

I sold my car last week. It's my biggest regret. I wish I kept my car.

The sentence means that the speaker wishes they had not sold their car. They regret selling the car and would like to have it back.

The word kept expresses the speaker's wish to maintain their possession of the car. It implies that the speaker valued the car and did not want to part with it.

The other options are not as fitting for the sentence.

1. bought: This word suggests that the speaker did not own the car before and decided to buy it. However, the speaker is expressing regret about selling a car that they already owned.

2. sold: This word is the opposite of kept and would contradict the meaning of the sentence.

3. given: This word suggests that the car was not owned by the speaker but was given to them by someone else. This is not the case in this sentence.

4. driven: This word suggests that the speaker used the car for transportation. However, the speaker is expressing regret about selling the car, not about driving it.

Therefore, the only fitting word is kept.