The word 'came' in the sentence indicates that the tense of the sentence is past tense. Thus, the answer must also be in the past tense.
Option A is incorrect. 'am' is in the present tense and not the past tense. Thus, using 'an' will make the tense of the sentence inconsistent.
Option B is incorrect. 'will be' is in the future tense and not the past tense. Thus, using it will make the tense of the sentence inconsistent.
Option C is correct. 'had been' is in the past tense. Thus, it has the correct tense and is the correct answer.
Option D is incorrect. 'won't' is short for 'will not' and so ,it is in the future tense and not the past tense. Thus, using it will make the tense of the sentence inconsistent.