ANS:A - Viscous force

The force that does not act on a fluid at rest is the viscous force. When a fluid is at rest, there is no relative motion between its layers, so there is no shear stress and thus no viscous force acting within the fluid. Gravity force, hydrostatic force, and surface tension force can still act on a fluid at rest:

1. Gravity force: Acts vertically downward, influencing the weight of the fluid and any objects within it.
2. Hydrostatic force: Arises due to the pressure difference between different points within the fluid, causing a net force in the direction of decreasing pressure.
3. Surface tension force: Exists at the interface between the fluid and another phase (such as air or a solid surface) and tends to minimize the surface area, resulting in a force tangential to the surface.