Which sentence is in passive voice?

1. They are writing a letter.
2. She is singing a song.
3. A letter is being written by them.
4. He plays football.

The passive voice emphasizes the action and the object that receives the action. The structure is [object] + [be verb] + [past participle] + “by” [agent]. In this case, “A letter” (object) receives the action “is being written”.