Two bodies A and B are said to be in thermal equilibrium with each other if
No heat flows between A and B.
Heat flows between A and B at a constant temperature.
The correct option is D No heat flows between A and B.
Two physical systems are in thermal equilibrium with each other, if there is no net flow of thermal energy between them when they are connected by a path permeable to heat.
Thus, option (c) is the correct answer.