Which sentence uses the passive voice?
a. the tree was planted by mary.
b. the rain continued until dawn.
c. lies destroy friendship.
d. john searched for a job.

Answer : A. The tree was planted by Mary. 

A sentence is said to be in the passive voice when the subject is not the doer of the action but the receiver of the action. The subject is being acted upon unlike the active voice of the verb wherein the subject is the doer of the action. Here are another set of sentences in its passive voice.

The cake was eaten by her sisters.
The big box was being carried by the huge man in the market.
The flower was being arranged by the beautiful lady.