How would you define drag force in fluid dynamics?

In fluid dynamics, drag force is defined as the resistance force experienced by an object as it moves through a fluid. This force acts in the opposite direction to the object's motion and is influenced by factors such as the object's speed, shape, and the properties of the fluid. As an object moves through a fluid, it must push the fluid out of the way, creating a wake behind it where the fluid moves back into place.

The magnitude of the drag force typically depends on the object's velocity relative to the fluid, the density of the fluid, and the drag coefficient, which represents how easily an object can move through a fluid based on its shape. Therefore, B is the most accurate definition of drag force in the context of fluid dynamics.

Other options do not correctly represent drag force. The force applied by a fluid at rest on an object refers more to pressure forces, the upward force acting against gravity describes buoyancy, and the total force applied to a stationary object does not capture the dynamics involved when the object is actually moving through a fluid.