Had I a car, I -fast.

The correct answer is:

a) Would come

Explanation:

The sentence, "Had I a car, I would come fast," is a conditional construction expressing a hypothetical situation in the past. In such cases, the correct form to use is "would" + base verb. The correct completion of the sentence is "Would come," making option (a) the right choice.

Explanation for why the other options are not correct:

b) "come": This doesn't match the conditional structure needed for the sentence.

c) "had come": This would be used in a different context, not the conditional structure presented in the original sentence.

d) "have come": This is a present perfect construction and doesn't fit the structure needed for the sentence describing a hypothetical past situation.