Select the correct passive form of the given sentence.

The police beat a number of protestors last night.

A number of protestors were beaten by the police the last night.

Active Voice: "The police beat a number of protestors last night." ⇒ the subject (the police) is performing the action (beating), and the object (a number of protestors) is receiving the action.

Passive Voice: "A number of protestors were beaten by the police last night." 

Note:

Passive voice: the object of the active sentence becomes the subject of the passive sentence. The verb "beat" changes to its passive form ("beaten"). The agent of the action (the police) is placed at the end of the sentence, introduced by the preposition "by."