The correct answer is D) A single zone system. This is the correct answer because the illustrated diagram GS-RA-09 shows a simple, single-zone HVAC system where air is supplied to a single space or zone. In a single zone system, there is a single thermostat that controls the temperature for the entire space, and the air handling unit provides a fixed volume of conditioned air to that zone. The other answer choices are incorrect because they describe more complex HVAC system configurations that are not depicted in the provided illustration. A dual duct system, terminal reheat system, and variable air volume system all involve multiple zones or more advanced controls, which are not shown in the single zone system diagram.