A thermostat serving a unit with separate heating and cooling control transformers and one stage of each is classified as a ... control.

A thermostat serving a unit with separate heating and cooling control transformers and one stage of each is classified as a DPDT (Double Pole Double Throw) control. This classification indicates that the thermostat has two separate circuits, allowing it to control both heating and cooling functionalities.

In a DPDT configuration, the device features two poles (which can control two independent circuits) and each pole can connect to one of two outputs, effectively providing the ability to switch between different states for both the heating and cooling systems. This is important because it allows a thermostat to switch between heating and cooling depending on the temperature setpoint, ensuring comfort and efficiency in various conditions.

In contrast, options like SPST or SPDT would not adequately manage separate circuits for heating and cooling since they are single-pole configurations. An SPST (Single Pole Single Throw) operates only one circuit, while an SPDT (Single Pole Double Throw) can switch between two paths but cannot handle multiple independent systems like a heating and cooling setup effectively. The DPST, while it manages two circuits, lacks the capability of changing between multiple outputs as required in this situation. Thus, DPDT is the most appropriate classification for the given setup.