The correct option is A wouldn't
Apostrophe can be used to show that a letter or letter(s) is missing from a word. It can be used to omit the letter or letter(s). In the given sentence, 'wouldnt' used is not correct. Here, apostrophe should be used to replace the letter 'o', and the two words (would and not) can be joined to make one word i.e wouldn't. Thus, option B is correct.