We have a linear function and we have to find the meaning of the slope.
The function is:
In this function the slope is m=50, as it is the coefficient for h, the number of hours.
The slope usually represents the variation of the result variable (in this case, the cost in dollars) and the independent variable (in this case, h, the number of hours).
Then, we can think of the slope in this model as the marginal hourly rate he charges. This means that any additional hour of work will cost $50 more.
Then, from the options given, the correct one is: the charge per hour [Option D].