The correct answer is $\boxed{\text{B}. $12,800}$.
The slope coefficient is the amount of change in the dependent variable (machine hours) for every unit change in the independent variable (cost). In this case, the slope coefficient is 0.40, which means that for every $1 increase in cost, there is a $0.40 increase in machine hours.
The difference in costs is $32,000. To find the difference in machine hours, we multiply the slope coefficient by the difference in costs. $0.40 \times $32,000 = $12,800.
Therefore, the difference in machine hours is $12,800.
Option A is incorrect because it is the total cost, not the difference in cost.
Option C is incorrect because it is the total machine hours, not the difference in machine hours.
Option D is incorrect because it is the difference in cost multiplied by the slope coefficient, not the difference in machine hours.