Technically, 'were' is correct but many native speakers would also say 'was', so this could be considered correct too. It is an unreal present/future conditional (2nd conditional), so you use 'would' in one part and the past subjunctive 'were' in the second part. For all verbs except 'be' the past subjunctive is the same as the past simple, so it has become normal to use the past simple of 'be' in conditionals. The past subjunctive actually tells us that the situation is unreal, rather than in the past, so this is why it appears that we use past tenses for present and future situations. You could use 'had been' in a past (3rd) conditional: We would have left immediately if Jane had been ready. It would be unusual to use 'would be in the 'if' part of the sentence. So b. is certainly correct and a. may also be correct depending on who is asking the question.

were