Answer: (d)Rotational speed of the shaft
Explanation: Shaft design is the design of a shaft that is used for defining the stresses at certain critical part of shaft. The shaft design has shaft diameter as a major part and this is determined by several factors like type of bearing , deflection, torque,safety factor etc.
But the least important factor for determining of the diameter is the rotational speed because it defines the rotation of an object around a particular axis, where is it states about the number of turns divisible by time. Therefore option(d) is the correct option.