The correct answer is D. Buoyant force is the resultant force on a body due to the fluid surrounding it. It is always directed upwards and is equal to the weight of the fluid displaced by the body.
Option A is incorrect because the buoyant force is not the resultant force acting on a floating body. The resultant force acting on a floating body is the sum of the buoyant force and the weight of the body.
Option B is incorrect because the buoyant force is not equal to the volume of liquid displaced. The buoyant force is equal to the weight of the fluid displaced by the body.
Option C is incorrect because the buoyant force is not the force necessary to keep a body in equilibrium. The buoyant force is one of the forces that act on a body in equilibrium, but it is not the only force. The other force that acts on a body in equilibrium is the weight of the body.