The correct option is
A weren't
The given sentence is in the subjunctive mood. This means that it expresses a condition that is doubtful or not factual. Further, the word 'would' indicates that the tense of the sentence is past tense. Thus, the answer must be in the past tense and must comply with the rules of subjunctive mood.
Option A is correct. 'weren't' is in the past tense and also complies with the rules of subjunctive mood. Thus, it is the correct answer.
Option B is incorrect. 'isn't' is in the present tense and not the past tense. Thus, using it will make the tense of the sentence inconsistent.
Option C is incorrect. 'wasn't' is in the past tense but, for subjunctive mood, the past tense conjugation of the verb 'to be' is weren't and not wasn't. Thus, 'wasn't' does not comply with the rules of subjunctive mood and so, it is incorrect.
Option D is incorrect. 'won't be' is short for 'will not be' and so, it is in the future tense and not the past tense. Thus, using it will make the tense of the sentence inconsistent.