The correct answer is B. have not sold.
The sentence is about the speaker’s current situation, which is that they have two cars. The speaker bought a new car last year, but they have not sold their old car yet. This means that they still have both cars.
The other options are incorrect because they do not accurately describe the speaker’s current situation. Option A, could not sell, suggests that the speaker tried to sell their old car but was unsuccessful. Option C, have sold, suggests that the speaker has already sold their old car. Option D, did not sell, suggests that the speaker has made a decision not to sell their old car.
In conclusion, the correct answer is B. have not sold.