Which of the following sentences is written in the passive voice?

A. They considered their options.

B. I would not provoke that man if I were you.

C. The effects of cyanide are well known.

D. Having left the building, Elvis never returned.

The sentence which is written in the passive voice is C. The effects of cyanide are well known. When there is no performing subject in the sentence and verb stands as a doer of an action, that means that verb is put in the passive. The rest of options contain both subject and verb, whereas in option C subject is performed by the verb.

We are to determine which among the given choices is an example of a sentence that is in its passive form. Sentences in passive form are those with doer of the action as the predicate. From the sentences presented, the answer is, "The effects of cyanide are well known". The answer is letter C.