The boy killed the snake with a stick.

The correct answer is A. The snake was killed by the boy with a stick.

B is incorrect because the stick is the instrument that was used to kill the snake, not the one who killed the snake.

C is incorrect because the snake and the stick are not combined into a single entity.

D is incorrect because the sentence is in the present tense, but the action of killing the snake has already happened.

The sentence “The boy killed the snake with a stick” can be paraphrased as “The boy used a stick to kill the snake.”