Which of the following principles relates to thermal equilibrium?

The correct answer relates to the concept of thermal equilibrium, which is defined by the Zeroth Law of thermodynamics. This law states that if two systems are each in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. This principle establishes the foundational idea that temperature is a fundamental property that determines the direction of heat transfer.

When two bodies are in thermal equilibrium, it signifies that there is no net flow of thermal energy between them, and thus, they are at the same temperature. This principle is essential for understanding concepts of heat transfer and temperature measurement in thermodynamics.

In contrast, the other principles listed do not directly relate to thermal equilibrium. Bernoulli's principle deals with fluid dynamics and the relationship between the pressure and velocity of a fluid. Charles's law pertains to the relationship between the volume and temperature of a gas at constant pressure, while Boyle's law addresses the relationship between the pressure and volume of a gas at constant temperature. These laws describe behaviors of gases but do not focus on the concept of thermal equilibrium as defined by the Zeroth Law.