Question

What is the most important reason for setting a purpose before reading

Answer 1

The answer is D: to concentrate on your personal areas of interest.

 The most importante reason for setting a purpose before reading or even before doing a research is to eliminate distraction, which can be a hindrance for any desired development. Having a purpose is to have an intention, a goal, which will be achieved only with concentration on one's personal areas of interest.

Answer 2

a. to provide a focus and thus avoid being overwhelmed 
if you do not set a path for yourself you may get sidetracked to prevent that you set a focus