The meaning of the slope in the context of the problem is,

The correct answer is: It costs $3 for each book.

The meaning of the slope in the context of the problem is it costs $3 for each book.
from graph we can see that line is passing through (5,30) and (15,60)
slope of line passing through (x1,y1) and(x2,y2) is y₂-y₁/x₂-x₁
slope=60-30/15-5
slope=3
slope =y/x=total expenses/number of book
so cost of 1 book will be $3