Hey there,
Your answer is A) To distract the reader from the main plot
Subplots are not used to distract the reader from the main plot. You want the reader to know the story and figure out the main plot, not pull them away from it. Subplots are used to develop the novels theme in more depth, to further develop conflict, and to further develop character. Not to pull the reader from the main plot.
Best Of Luck,
- I.A. -