In an isolated system where two substances are put into contact, the energy in the system will transfer between the substances until thermal equilibrium is reached. Based on the options provided, the most accurate statement would be:
The energy in the system will transfer from substance A to B until equilibrium is reached.
or
The energy in the system will transfer from substance B to A until equilibrium is reached.
The correct choice depends on which substance has a higher initial energy (or temperature). If substance A has a higher temperature, energy will transfer from A to B, and if B has a higher temperature, then energy will transfer from B to A. In either case, the total energy in the isolated system remains constant; it is just redistributed between the two substances.