Just as books have chapters, plays are divided up into

a. acts.

b. scenes.

c. verses.

d. lines.

Plays are divided into acts. Depending on the story the number of acts can be two, six or even more. The time of an act can be from thirty to ninety minutes. But it can also be very short, for example fifteen minutes. Acts are also used for give theatregoers and actors the break. Acts are divided into scenes. There are changing of characters in every scene, because because often the same actors play all the roles.

The answer is acts.