In order to select the correct form of the verb to complete the sentence, let's understand the subject-verb agreement. Subject and verb should agree with each other, i.e., if the subject is singular, the verb should also be in its singular form; and if the subject is plural, the verb should also be in its plural form.NOTE: 'Neither-nor' is a conjunction, which means none of the subjects/nouns/pronouns.Also note: In case of 'either-or' or 'neither-no', if one of the subjects is plural, the verb should be in its plural form as well.Let's have a look at the options:Option B - 'Was' is a singular form of verb, used with a singular subject. Though the conjunction 'neither-nor' usually have one subject, the subject is plural here. As explained above, if one of the subjects, especially the one before the verb is plural, then the verb has to be plural too. Hence, option B is incorrect.Option C - 'Is' is a singular form of verb, used with a singular subject. As explained above, the subject in the given sentence plural, hence, we can't use 'is' here. Thus option C is incorrect.Option D - 'Been' can be used as both singular as well as plural form of the verb. It can be used with singular as well as plural subject. However, 'been' is accompanied by a helping verb to make the sentence grammatically correct. It is often used in perfect and perfect continuous tenses. Hence, option D is incorrect. Option A - 'Were' is a plural form of verb, which is used with plural subject. As the second subject in the sentence is plural, we can use 'were' here. Hence, we can conclude 'Neither the teacher nor the students were in the classroom' is grammatically correct. Thus option A is the correct answer.