The correct answer is C. high.
The design of mechanical parts is particularly important in case of high speed machines because the forces and stresses involved are much greater than in low speed machines. This means that the parts need to be designed to be stronger and more durable. In addition, the tolerances for the parts need to be much tighter in order to ensure that they fit together properly and function correctly.
For example, the crankshaft in a car engine is a high speed machine. It rotates at speeds of up to 6,000 revolutions per minute (RPM). The crankshaft is made of a strong alloy steel and is machined to very tight tolerances. This is necessary to ensure that it can withstand the high stresses and forces that are generated during engine operation.
If the crankshaft were not designed properly, it could fail, which would cause the engine to stop running. This could be a very serious problem, as it could lead to an accident.
Therefore, the design of mechanical parts is particularly important in case of high speed machines. The parts need to be designed to be strong, durable, and accurate in order to ensure the safety and reliability of the machine.
The other options are incorrect because they do not take into account the specific requirements of high speed machines.
Option A, low speed machines, does not require the same level of design precision as high speed machines. The forces and stresses involved are much lower, so the parts do not need to be as strong or durable. In addition, the tolerances for the parts can be looser, which makes the design process less complex.
Option B, medium speed machines, falls somewhere in between low speed and high speed machines. The design requirements for medium speed machines are more stringent than for low speed machines, but not as stringent as for high speed machines.
Option D, any of the above, is incorrect because it does not specify which type of machine is being discussed. The design requirements for different types of machines can vary significantly, so it is important to specify the type of machine when discussing the design of mechanical parts.