Explanation:
In a roof truss system, purlins are horizontal structural members that support the roof covering and transfer the load to the main rafters or trusses. The correct answer to the given question is option 'D', which states that purlins are placed at the panel points to avoid bending moment in the rafters.

Reasoning:
When a load is applied to a roof truss, it creates different types of forces and moments on the various members. The location of purlins is crucial in order to minimize the bending moment in the rafters. Let's understand why placing purlins at the panel points helps in achieving this:

1. Definition of Panel Points:
Panel points are the locations in a truss where the individual members meet. These points are critical for load transfer and member stability.

2. Bending Moment in Rafters:
Bending moment is a measure of the bending effect caused by external loads on a structural member. It is the product of the force applied to the member and the distance from the force to the point of interest. In the case of roof trusses, the rafters are subjected to bending moments due to the applied loads.

3. Load Transfer:
When purlins are placed at the panel points, they act as intermediate supports for the roof covering. By distributing the load from the roof covering to the panel points, the purlins help in transferring the load to the main rafters or trusses.

4. Bending Moment Distribution:
When purlins are placed at the panel points, the load is transferred more evenly to the rafters. This distribution of load reduces the bending moment experienced by the rafters and helps in minimizing the risk of structural failure.

5. Axial Force, Shear Force, and Deflection:
While axial force, shear force, and deflection are important considerations in roof truss design, the primary purpose of placing purlins at the panel points is to avoid bending moment in the rafters. However, it is worth noting that the correct placement of purlins can also help in reducing shear force, axial force, and deflection in the rafters to some extent.

Conclusion:
To summarize, purlins are placed at the panel points in roof truss systems to avoid bending moment in the rafters. This placement helps in distributing the load more evenly and reducing the risk of structural failure. While other factors such as shear force, axial force, and deflection are also important, the primary objective is to minimize the bending moment in the rafters.