The correct answer is $\boxed{\text{A}}$.
The slope coefficient is a measure of how much the dependent variable changes in response to a one-unit change in the independent variable. In this case, the dependent variable is cost and the independent variable is machine hours. The slope coefficient is 0.75, which means that for every one-unit increase in machine hours, cost increases by 0.75 units.
The difference in machine hours is 65,000. Therefore, the difference in cost is $0.75 \times 65,000 = $86,667.
Option B is incorrect because it is the difference in cost for a one-unit change in machine hours. Option C is incorrect because it is the difference in cost for a 100-unit change in machine hours. Option D is incorrect because it is the difference in cost for a 50-unit change in machine hours.