The correct option is
C would
In the sentence, the word 'had' indicates that the tense of the sentence is past tense. Thus, the answer must be in the past tense.
Option A is incorrect. 'will' is in the future tense and not the past tense. Thus, using it will make the tense of the sentence inconsistent.
Option B is incorrect. 'won't' is short for 'will not'. Thus, it is in the future tense and not the past tense. Thus, using it will make the tense of the sentence inconsistent.
Option C is incorrect. 'were' is in the past tense but, were can only be used with plural nouns. Since the noun in the sentence is singular, using 'were' will make the sentence grammatically incorrect.
Option D is correct. 'would' is in the past tense and is also, it can be used for singular nouns. Thus, it is the correct answer.