Two system are said to be in thermal equilibrium if and only if:

1. there can be a heat flow between them even are at different temperatures
2. there cannot be a heat flow between them even if they are at different temperatures
3. there is no heat flow between them
4. their temperature are slightly different

Two physical systems are in thermal equilibrium if no heat flows between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics.

The correct option is C.