Rectangular sections are rarely used in beams because more material in kilos is required.

Explanation:

Uniform Stresses:
One reason why rectangular sections are rarely used in beams is because the stresses are not uniform in a rectangular section. In a rectangular beam, the top and bottom portions experience greater stress compared to the middle portion. This non-uniform stress distribution can lead to inefficient use of the material and may result in structural failure.

Material Requirement:
Rectangular sections require more material in kilos compared to other beam sections. This is because the rectangular shape does not distribute the load efficiently, leading to higher stress concentrations at the corners. The excess material adds unnecessary weight to the beam, making it less cost-effective and more cumbersome to handle during construction.

Plastic Stresses:
Contrary to option 'C', stresses in a rectangular section are not always plastic. Plastic stresses refer to the deformation of a material beyond its elastic limit, which can occur in certain situations. However, the choice of beam section does not determine whether the stresses will be plastic or not. It depends on the loading conditions, material properties, and design considerations.

Efficiency:
Using a rectangular section for a beam is not an efficient choice due to the non-uniform stress distribution and the requirement for more material. Other beam sections, such as I-beams or H-beams, are commonly used because they are more structurally efficient. These sections distribute the load more evenly and require less material to achieve the same strength and stiffness.

Conclusion:
In summary, rectangular sections are rarely used in beams due to the non-uniform stress distribution, the requirement for more material in kilos, and the availability of more efficient beam sections. It is important to consider the structural requirements, cost-effectiveness, and material efficiency when selecting a beam section for a specific application.