The paragraph refers to simple past tense.
"Is" in option A is the third person singular present of the auxiliary verb 'be'. This is incorrect.
"Are" in option B is second person singular present and first, second, third person plural present of the auxiliary verb 'be'.
"Had" is incorrect as it renders the sentence ungrammatical.
"Were" in option D is correct as the second person singular/plural past subjunctive of the verb form of the auxiliary verb 'be' indicates the different sections that existed.
Thus, option D is correct.