In the given sentence, the present participle form of the main verb i.e 'raining' is already given. Only the auxiliary verb- 'was' or 'were' has to be added.
Option A is incorrect because, 'were' is the plural form of the verb 'to be'. However, the pronoun, 'it' is singular. Hence, 'were' cannot be used.
Option B is correct because, 'was' is the singular form of the verb 'to be' which can be used with the singular pronoun 'it'.
Option C is incorrect because, the blank has to be filled with either 'was' or 'were' for the past continuous form. 'Will be raining' indicates future continuous tense.
Option D is incorrect as option B is the correct answer.