An A/C system with zone control that mixes hot and cold air in response to room thermostats is a _____ system?

A multi-zone system is designed to provide different temperature control in various areas or zones of a building based on individual room thermostats. This capability allows the system to mix hot and cold air as required to maintain the set temperature in each room, adapting to different heating and cooling needs.

Multi-zone systems typically feature multiple zones that can be independently controlled, enabling better energy efficiency and occupant comfort. By using dampers to regulate air flow and adjusting the system's output based on the signals from the individual thermostats, the system ensures that each zone maintains its desired temperature without unnecessary over-cooling or heating.

The other options do not fit this function. A single-zone system serves only one temperature-controlled area, variable-zone systems are generally considered a less common terminology, and dual-zone usually implies only two temperature-controlled areas rather than a system that can handle multiple zones independently. Thus, the answer aligning with a system that responds to multiple thermostats is appropriately the multi-zone system.