According to the zeroth law, when are two systems considered to be in thermal equilibrium?

Two systems are considered to be in thermal equilibrium according to the zeroth law when they are in thermal contact and their variables do not change. This situation indicates that there is no net exchange of thermal energy between the two systems, which means that they have reached the same temperature.

When two systems reach thermal equilibrium, it means that if one system were to be brought into contact with the other, there would be no energy flow between them; they would share the same thermal state. The zeroth law establishes a foundational understanding of temperature and thermal interactions. It implies that if system A is in thermal equilibrium with system B, and system B is in thermal equilibrium with system C, then system A and system C are also in thermal equilibrium with each other.

In contrast, the other provided choices do not correctly define thermal equilibrium. Differences in pressure or internal energy do not guarantee equilibrium, nor does merely exchanging energy if they haven't reached the same temperature yet. Therefore, the condition regarding thermal contact and unchanged variables effectively signifies that thermal equilibrium has been achieved.