Which sentence cannot be made passive? 

This question asks which sentence cannot be turned into a passive construction. Passive voice is when the subject of a sentence is no longer the “doer” of the action, and instead, the object of the sentence becomes the subject. For example, “I ate the sandwich” can be turned into “The sandwich was eaten by me” in a passive voice. Out of the four sentences given, “A tree stood here” cannot be made passive because it doesn't have an object that can be acted upon by a subject. The other three sentences have clear subjects performing actions on objects which can be restructured to form a passive voice construction.