The correct answer is: C. 1000 to 1500 r.p.m.
Commercially available medium size machines have a speed range of 1000 to 1500 r.p.m. This is because these machines are designed to be used for a variety of tasks, and they need to be able to operate at a variety of speeds in order to be effective. For example, a machine that is used to cut wood may need to operate at a high speed in order to make quick cuts, while a machine that is used to grind metal may need to operate at a lower speed in order to produce a smooth finish.
The other options are incorrect because they do not represent the typical speed range of commercial available medium size machines. Option A is too low, and option B is too high. Option D is also incorrect because it is not a typical speed range for any type of machine.
In conclusion, the correct answer is: C. 1000 to 1500 r.p.m.