Answer:
C. the rate at the pool fills in feet per hour
Explanation:
Given
The attached graph
Required
What does the slope represent?
The slope of any graph can be calculated using:

Where:
Rise = The change in the vertical axis
Run = The change in the horizontal axis within the same interval as the Rise
The vertical axis measures depth in feet and the horizontal axis measures time in hour.
So:

or

This means that the slope measures rate in feet per hour