The correct answer is C. 25.
The velocity ratio of a simple screw jack is the ratio of the distance the screw moves to the distance the handle moves. It is calculated by dividing the pitch of the screw by the length of the handle.
In this case, the pitch of the screw is 9 mm and the length of the handle is 45 cm. Therefore, the velocity ratio is 9 mm / 45 cm = 0.02 mm/cm.
To convert this to a more convenient unit, we can multiply it by 100 to get 2 mm/m.
Therefore, the velocity ratio of the system is 25.
Option A is incorrect because it is the ratio of the circumference of the screw to the length of the handle. This is not the same as the velocity ratio.
Option B is incorrect because it is the ratio of the diameter of the screw to the length of the handle. This is also not the same as the velocity ratio.
Option D is incorrect because it is the number of threads per inch on the screw. This is not the same as the velocity ratio.