An action in a sentence can be represented in two ways, namely active and passive voice.

The passive voice always uses the past participle form of the main verb irrespective of any tense. Only the auxiliary verbs depend upon the sentence given in active voice.

The correct answer is B)was thought to be as the previous sentence indicates simple past tense. (passive)

Options A, C, and D are not correct as the sentence is not in the past continuous tense, simple present tense, and helping verb is not used respectively.

Long ago, people thought that the earth was flat. Long ago, the earth was thought to be flat.