Final answer:
The water in the bottle absorbs heat from the sauna to reach the same temperature, making it an endothermic process.
Step-by-step explanation:
When George takes a bottle of water at 21°C into a sauna at 65°C, the water in the bottle will absorb heat from its surroundings until it reaches thermal equilibrium with the sauna's air temperature. This process is endothermic because it involves the absorption of heat from the surroundings, similar to how water needs heat to boil on a stove. An exothermic process, in contrast, releases heat into the surroundings as in the burning of a log in a campfire. Therefore, the correct answer is: a) Endothermic because the water absorbs heat from the surroundings.