You should test your brakes when you are not moving too fast. The answer to this question is B. When your vehicle is moving at 5 MPH. This is the most obvious answer because this is when you will realize if there is something wrong with your brakes. Your speed is slow enough that you can still stop on your own if in case your brakes are not working.
It is ideal if you are going to brake at Brake Testing Centers but this is not always available. You do not have the time to go to the center just to test your brakes, right? You should not test your brakes when you are going down a hill because if your brakes are not working well, you would surely crash into the first solid thing that you see.