In option A, "A stone" is the subject, which performed the action -"struck". AS the subject did the action, this sentence is in active voice.
In option B, the "The ball" is the subject. Here, the subject does not perform the action. It is the receiver of the action. It "was caught" by the child. i.e An action was performed on the subject -"the ball". Hence, this sentence is in passive voice and not the correct answer.
In option C, "I" is the subject. Here, the subject does not perform the action. It is the receiver of the action. We know this because of the words, "I was caught unaware". i.e An action was performed on the subject -"I". Hence, this sentence is in passive voice and not the correct answer.
In option D, the word, "he" is the subject. Here, the subject does not perform any action. The sentence says-He "was cheated" by his own wife i.e "He", the subject is the receiver of the action. Hence, this sentence is in passive voice and is not the correct answer.