The correct answer is B. To help the viewer's eye travel from one area to another.
A painting can be a dynamic artwork, even though all the elements are present at the same time, and nothing changes. However, when they depict an event of some kind, not all the people are still and unmoving. For example, if someone points a finger at something, we will necessarily look for it, and thereby unravel the "plot" of the painting.
Here are the reasons why all the other answers are incorrect. A painter doesn't plan to leave his work unfinished. Basically, there are no unimportant areas in a painting. There is only so much space on the canvas, and an artist makes sure for every element to have a function. Also, artists usually don't intend to divert their viewers' attention to things outside the painting.