The correct answer is: A. Displacement
The virtual work method is a technique used in structural analysis to calculate the internal forces and reactions in a structure. The method is based on the principle of virtual work, which states that the work done by the external forces on a structure is equal to the work done by the internal forces on the structure.
In the virtual work method, the virtual quantity is the virtual displacement. The virtual displacement is a small, imaginary displacement that is applied to the structure. The work done by the external forces on the structure due to the virtual displacement is calculated, and this work is equal to the work done by the internal forces on the structure due to the virtual displacement.
The virtual work method can be used to calculate the internal forces and reactions in any type of structure, including beams, frames, and trusses. The method is relatively simple to use, and it can be used to calculate the internal forces and reactions in structures that are difficult to analyze using other methods.
The other options are incorrect because they are not the virtual quantity in the virtual work method. Load is a force that is applied to a structure, and slope is the change in the angle of a line. Moment is a force that causes a rotation about a point.