ANS:B - viscous force exerted by air.

Viscous force exerted by air refers to the resistance encountered by an object moving through the air due to the viscosity, or thickness, of the air itself. Air is not a perfect vacuum; it has substance and viscosity, albeit much lower than that of liquids. As an object moves through the air, it displaces air particles along its path. The air particles in contact with the surface of the object tend to stick to it due to viscosity, creating a thin layer of air that moves with the object. This layer of air, known as the boundary layer, experiences shear stress as the object moves through it. The shear stress arises from the difference in velocities between adjacent layers of air. The faster-moving outer layers of air tend to drag along the slower-moving layers closer to the object's surface, creating resistance to the object's motion. The magnitude of the viscous force depends on several factors, including the velocity of the object, the density and viscosity of the air, and the shape and surface characteristics of the object. For example, streamlined shapes encounter less viscous drag compared to blunt shapes due to differences in how they interact with the air molecules. In the context of raindrops falling through the atmosphere, the viscous force exerted by the air opposes the motion of the raindrop, eventually reaching equilibrium with the gravitational force acting on the raindrop. This equilibrium results in a constant terminal velocity, where the downward gravitational force is balanced by the upward viscous drag force, allowing the raindrop to fall at a steady speed.