The y-intercept of the graph shows that at m = 0, c = 15. We can see a constant slope that causes the graph to pass through m = 20, c = 20, as well as m = 40, c = 25. This shows a constant increase of $5/20 minutes, or a slope of 5/20 = 1/4. Based on slope-intercept form y = mx+b, the equation would be c = (1/4)m+15, from 0 <= m <= 44,640. This is the second of the given choices.
