What indicates that a process has reached thermal equilibrium?

A process reaches thermal equilibrium when there are no temperature gradients within the system, meaning all parts of the system have the same temperature. This uniformity in temperature indicates that energy is no longer being exchanged as heat between different parts of the system. Additionally, at thermal equilibrium, the pressures can also equalize if applicable, leading to uniformity in pressure as well.

No net heat transfer occurs at this stage because the system's internal energy is stable; thus, there are no temperature differences to drive heat exchange.

When combining all these aspects – having the same temperature, potentially the same pressure (in a closed system), and no net heat transfer – it can be concluded that these interconnected factors collectively define thermal equilibrium. Therefore, since all these criteria must be met for a process to be in thermal equilibrium, the comprehensive answer is that all of the options presented are correct.