The correct answer is B) 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 provide independent heating and cooling to different zones within a building simultaneously. This is achieved by having separate supply and return pipes for the heating and cooling circuits, allowing the system to heat some areas while cooling others based on individual zone demands. In contrast, a 2-pipe system can only provide either heating or cooling at a given time, as the same pipes are used for both functions. The other options are incorrect because A) a 4-pipe system does not require double the amount of piping, C) it does not serve twice as many zones, and D) it requires more piping, not less, compared to a 2-pipe system serving the same number of zones.