Which of the following sentences is written in the passive voice?
a. i would not provoke that man if i were you.
b. having left the building, elvis never returned.
c. the effects of cyanide are well known.
d. they considered their options.

I don't mean to offend the first respondent, but her answer was incorrect.

I am 1,000,000,000% certain that this is the correct answer.

C. The effects of cyanide are well known.

Passive voice indicates that the subject of the sentence (effects of cyanide) is not the source of the action.

We can turn this example sentence around to say: "[Most people] know well the effects of cyanide."

We cannot do the same with the other sentences. All their subjects are the source of the action.

Atahualpaemperor of the inca empire who rose to the throne after winning a civil war between him and his brother.  -he refused to become a spanish vassal or christian.-francisco pizarro kidnapped him and demanded a huge ransom.  -pizarro's secretary described him as "a man of thirty years, good-looking and poised, somewhat stout, with a wide, handsome, and ferocious face, and the eyes flaming with blood."-after the ransom was paid, atahualpa was killed by pizarro