The shear diagram will jump downward if the force acts downwards. Let's understand why this is the correct answer in detail:

Shear Diagram:
A shear diagram is a graphical representation of the shear force along the length of a beam. It helps us to visualize how the shear force changes at different points along the beam.

Understanding Shear Force:
Shear force refers to the internal force that acts parallel to the cross-section of a beam. It is generated as a reaction to the external forces applied to the beam, such as point loads, distributed loads, or moments.

Explanation:
When a force acts downwards on a beam, it creates a positive shear force above the point of application and a negative shear force below it. This is because the force tends to cause the upper part of the beam to slide downwards and the lower part to slide upwards, resulting in opposing internal forces.

The shear diagram shows the distribution of these internal forces along the length of the beam. When a downward force is applied, the shear force starts from zero at the left end of the beam and gradually increases as we move towards the point of application of the force. At the point of application, the shear force jumps downwards abruptly, indicating the change in the internal forces.

Reasoning:
- Option A: Rotational moment does not directly affect the shear force. It is related to the bending moment and the curvature of the beam.
- Option B: Bending moment affects the shear force, but it does not cause a sudden jump downwards in the shear diagram.
- Option C: Total weight is a distributed load acting on the beam. It does not cause a sudden jump in the shear force.
- Option D: When a force acts downwards, it causes a sudden jump downwards in the shear force at the point of application. This is the correct answer as it explains the behavior observed in the shear diagram.

Conclusion:
In summary, the shear diagram will jump downward if a force acts downwards on the beam. This is because the downward force creates opposing internal forces that cause a sudden change in the shear force at the point of application.