Complete the sentence that best fits the blank:
He was thought [fill-regular] the disease when he was in Bangladesh.

Tenses denote the time an action takes place. Tenses is divided into three parts 1) tense 2) past and also the 3) future. Each of those can further be divided into 4 subparts 1) Simple 2) Continuous 3) Perfect 4) Perfect imperfect tense.
The given sentence is within the tense.

Let us discuss the given options one by one.
>Option A) to catch
This option is wrong because the sentence is within the tense and to catch is that the tense.

>Option B) catching
This option is wrong because the sentence is within the past and catching is within the present progressive tense.
>Option C) to own caught
This option is correct because it satisfies the principles of grammar and follows the tense of the tense which is that the tense. the solution is to possess as we use the infinite sort of the verb ( to + root verb ) when one verb is already conjugated was.
>Option D) to caught

This option is additionally incorrect because the phrase isn't grammatically correct.

Hence option C is correct.

Note: Rules of Tenses help one understand the way to correctly use the various tenses during a sentence. Hence it's important to grasp the foundations of the tenses thoroughly.