Final answer:
The final temperature at thermal equilibrium depends on initial temperatures, the specific heats, and the masses of the substances. It is reached when there is no net heat transfer between the substances and is typically between the higher and lower initial temperatures.
Step-by-step explanation:
The final temperature of both substances at thermal equilibrium is not determined by any of the options given (average, higher or lower of the initial temperatures). The actual final temperature depends on several factors including the initial temperatures of the substances, their specific heats, and their masses.
Thermal equilibrium is reached when the two substances have the same temperature, meaning there is no net heat transfer between them. This final equilibrium temperature will be somewhere between the initial temperatures of the two substances, rather than simply their average or one of the initial temperatures.
The direction of heat transfer is always from the higher-temperature object to the lower-temperature object until equilibrium is achieved.