Here's why:
* Passive Voice: The subject of the sentence is the thing being acted upon (the rose), and the verb "smells" is in the passive form.
* Active Voice: The active voice would be "The rose smells [something]," where the rose is the one performing the action of smelling.
To make the sentence active, you could say:
* The rose has a pleasant scent.
* The rose emits a sweet fragrance.