The correct answer is A. $16,000.
The slope coefficient is the change in the dependent variable (cost) for a one-unit change in the independent variable (cost driver). In this case, the independent variable is machine hours and the dependent variable is cost. The slope coefficient is calculated by dividing the difference between the highest and lowest observations of cost by the difference between the highest and lowest observations of cost driver. In this case, the difference between the highest and lowest observations of cost is $8000 and the difference between the highest and lowest observations of cost driver is 40 machine hours. Therefore, the slope coefficient is $16,000.
Option B is incorrect because it is the difference between the highest and lowest observations of cost. Option C is incorrect because it is one-half of the slope coefficient. Option D is incorrect because it is twice the slope coefficient.