Lilly took her brother Joe to the playground. Another little boy arrived at the playground with his mother. The boy started playing with Joe. The teeter-totter was shared by the boys. Which sentence is in passive voice?

The teeter-totter was shared by the boys.

Explanation:

Passive voice is when the object of the sentence becomes the subject of the sentence. The subject of the sentence then receives the action instead of doing it. Passive voice always includes a form of the verb 'to be' and a particle of the main verb. In the case of the last sentence, the teeter-totter is the object/subject. "Was" is the past tense form of the verb "to be." Shared is the past particle of the verb "share."

So basically, the breakdown is like this:

The teeter-totter  +   was       +   shared by   +  the boys.

       object          past tense    past particle      subject

                            of the verb    of the verb

                               'to be'             'share'

All the other sentences are in active voice as the subject is doing the action. "Lilly took", "Little boy arrived", "The boy started playing" all have the subject in front of the verb.  

Hope this helps. :)