y is the cost and x is the time.
In the graph, the shaded region is above the line, meaning that y has to be greater than x, and as the line is continuos, y is greater than or equal to x.
Finally, as we can see, when x = 0 then y = 4, meaning that the intersection with the y-axis is at number 4.
Answer: D. The cost is greater than or equal to 8 times the number of hours she rents the boat, plus $4