Explanation:

To understand why the moment diagram for a cantilever beam subjected to a bending moment at the end of the beam is a rectangle, we need to first understand the concept of a moment diagram and how it relates to the bending moment in a beam.

Moment Diagram:
A moment diagram is a graphical representation of the variation of bending moment along the length of a beam. It shows the magnitude and direction of the bending moment at different points along the beam.

Bending Moment:
When a beam is subjected to a bending moment, it experiences internal forces that cause it to bend. The bending moment is a measure of the intensity of these internal forces at a particular point on the beam.

Cantilever Beam:
A cantilever beam is a type of beam that is supported at one end and free at the other end. In the case of a cantilever beam subjected to a bending moment at the end of the beam, the bending moment is maximum at the support and decreases linearly towards the free end.

Rectangular Moment Diagram:
In a cantilever beam subjected to a bending moment at the end of the beam, the moment diagram is a rectangle. This is because the bending moment is constant along the length of the beam. Since the bending moment is maximum at the support and decreases linearly towards the free end, the moment diagram is a rectangle with its height equal to the maximum bending moment and its width equal to the length of the beam.

The moment diagram being a rectangle indicates that the bending moment is constant along the length of the beam. This means that the internal forces causing the beam to bend are evenly distributed along the beam.

So, the correct answer is option 'B' - rectangle.

B