(a) A stone struck the child on her right eye.
(b) The ball was caught by the child.
(c) I was caught unaware by his naughty tricks.
(d) He was cheated by his own wife.
Active voice means that a sentence has a subject that acts upon its verb. Passive voice means that a subject is a recipient of a verb’s action. In the given sentences, option (a) is in the active voice. The preposition ‘by’ is used in passive voice.
Hence option (a) is the answer.