The correct answer is: C. It was known to him that his own life was unjust.
The sentence “He knew that his own life was unjust” is in the active voice. In the active voice, the subject of the sentence performs the action. In this case, the subject is “he” and the action is “knew.” The object of the sentence is “that his own life was unjust.”
The passive voice is formed by using the verb “to be” and the past participle of the main verb. In this case, the passive voice sentence would be “It was known to him that his own life was unjust.” In the passive voice, the subject of the sentence receives the action. In this case, the subject is “it” and the action is “known.”
The passive voice is often used when the identity of the person or thing performing the action is not important or when the focus is on the receiver of the action. In this case, the identity of the person or thing who knew that his own life was unjust is not important. The focus is on the fact that he knew that his own life was unjust.
The other options are incorrect because they are in the active voice. In the active voice, the subject of the sentence performs the action. In these cases, the subjects are “it” and “him.” However, the focus of the sentence is on the receiver of the action, not the person or thing performing the action.