The bending moment at a section in a beam is a measure of the bending or twisting effect on the beam at that particular location. It is an important parameter in structural analysis and design as it determines the strength and stability of the beam.

The correct answer to the question is option 'D': the net moment exerted by the part of the beam on one side of the section to that on the other side of the section. This means that the bending moment at a section in a beam is equal to the difference in the moments exerted by the forces on either side of that section.

Here is a detailed explanation of why option 'D' is the correct answer:

1. Definition of bending moment: The bending moment at a section in a beam is defined as the algebraic sum of the moments of all the forces acting on one side of the section with respect to the section.

2. Moment of a force: The moment of a force about a point is the product of the force magnitude and the perpendicular distance from the point to the line of action of the force. It represents the ability of the force to cause rotation about the point.

3. Summation of moments: When we consider the bending moment at a section in a beam, we need to take into account all the forces acting on one side of the section. This includes both the external forces and the internal forces within the beam.

4. Internal forces: The internal forces in a beam, such as shear forces and axial forces, also contribute to the bending moment at a section. These forces result from the deformation of the beam under the applied loads and are distributed along the length of the beam.

5. Net moment: The net moment at a section is the algebraic sum of the moments exerted by all the forces on one side of the section. This net moment represents the bending effect on the beam at that section.

6. Equilibrium condition: In order for a beam to be in equilibrium, the sum of the moments about any point in the beam must be zero. This means that the bending moment at any section in the beam is equal to the net moment exerted by the forces on either side of that section.

In conclusion, the bending moment at a section in a beam is given by the net moment exerted by the part of the beam on one side of the section to that on the other side of the section. This includes both the external forces and the internal forces within the beam. Understanding the bending moment is crucial for analyzing the structural behavior of beams and designing them to withstand the applied loads.