How can you tell if two objects are in thermal equilibrium? a. the objects are touching each other. b. the object's temperatures are changing. c. the object's are the same size. d. the object's have the same temperature.

The answer is D, The objects have the same temperature. 

Usually, if two objects are touching each other, the one with the higher temperature may transfer heat to the other object by a method called conduction. During this process, the one with the higher temperature loses heat, while the cooler one gains heat. Their temperature will change until they both have the same temperature, and this can be called thermal equilibrium. 

As in "equilibrium", this word means same/balanced, and "thermal" is heat, so size is not related, and changing temperature also does not match with these 2 words. Therefore, the answer should be D.