Which of the following statements about liquid dynamics is true?

The statement that fluids can exert forces on objects in motion is true because of the fundamental properties of fluids and the forces they generate. When an object moves through a fluid, the fluid's viscosity and pressure can create drag or lift forces, depending on the conditions. For example, as a solid object moves through water or air, it displaces the fluid, resulting in a reaction force from the fluid that acts on the object. This interaction is essential in various applications, such as the design of aircraft wings, boat hulls, and even in understanding how parachutes work.

The other options do not hold true for fluids in their general behaviors. Fluids do exhibit viscous behavior, meaning they resist flow due to internal friction. Fluids, such as liquids and gases, can exist in different states of matter depending on the temperature and pressure. Lastly, while some fluids can be incompressible under certain conditions (like liquids), not all fluids, particularly gases, are incompressible. Thus, the correct statement about liquid dynamics is that fluids can exert forces on objects in motion.