The correct answer is C) A 4-pipe hydronic heating/cooling system allows simultaneous heating and cooling of different zones, whereas a 2-pipe hydronic heating/cooling system does not. The key advantage of a 4-pipe system is its ability to independently supply heating and cooling to different zones at the same time. This is because the 4-pipe system has separate supply and return lines for both the heating and cooling circuits, allowing them to operate independently. In contrast, a 2-pipe system has a single supply and return line, which means it can only provide either heating or cooling at a time, not both simultaneously. The other options are incorrect because they do not accurately describe the key advantage of a 4-pipe system over a 2-pipe system. Option A is incorrect because the number of zones served is not directly related to the number of pipes. Option B is incorrect because a 4-pipe system does require more piping than a 2-pipe system, not double. Option D is incorrect because a 4-pipe system requires more piping, not less, compared to a 2-pipe system.