When is the passive voice a useful tool in writing?
a. when the person doing the action is known
b. when several actions take place at the same time
c. when the focus of the sentence is on the cause of an action
d. when a writer wants to emphasize that someone is receiving an action, not doing it

Passive voice is sentence construction because the subject is placed to the end of the sentence and thus acted on rather than acting. For example, the dog bit the boy in passive should be, the boy was bitten by the dog.
Passive voice is a useful tool in writing when the writer wants to emphasize that someone is receiving an action and not doing it. For example, Cyrus repaired the bus here bus is the object. In passive voice it should be the bus was repaired.