The correct answer is option 'B': Maximum, minimum.

Explanation:

In mechanical engineering, when designing a shaft, it is common practice to have stepped diameters at different sections of the shaft. This is done for various reasons, including strength, functionality, and ease of manufacturing.

Stepped Diameter:
A stepped diameter refers to a change in diameter along the length of the shaft. The shaft is not uniformly cylindrical but has sections with different diameters. These stepped sections can have different diameters at the middle portion and the shaft ends.

Reasons for Stepped Diameter:
There are several reasons why a shaft may have stepped diameters:

1. Strength: By increasing the diameter at certain sections, the shaft can be made stronger to withstand higher loads and prevent failure. The larger diameter provides more material cross-section and increases the shaft's ability to resist bending, torsion, and other forces.

2. Functionality: Stepped diameters can be used to accommodate different components or to provide specific functionality. For example, in a rotating shaft with a gear or pulley, a stepped diameter may be used to provide a larger surface area for attaching the component securely.

3. Manufacturing: Stepped diameters can simplify the manufacturing process. It may be easier and more efficient to machine a stepped diameter rather than a constant diameter along the entire length of the shaft.

Choice of Stepped Diameter:
The question specifically asks for the diameter at the middle portion to be maximum and the diameter at the shaft ends to be minimum. This choice is based on the engineering principles mentioned earlier.

1. Maximum Diameter at Middle Portion: The maximum diameter at the middle portion provides increased strength and stability to the shaft. This is important because the middle portion of the shaft is usually subjected to higher loads and bending moments.

2. Minimum Diameter at Shaft Ends: The minimum diameter at the shaft ends serves to reduce unnecessary weight and material, making the shaft lighter and more efficient. The shaft ends typically have lower load requirements compared to the middle portion.

Conclusion:
To summarize, a shaft is commonly stepped with a maximum diameter at the middle portion and a minimum diameter at the shaft ends. This design choice enhances the strength, functionality, and manufacturing process of the shaft.