Which of the following refers to a heating system that can operate using electrical resistive elements?

The choice that refers to a heating system able to operate using electrical resistive elements is the electric heater. Electric heaters are designed specifically to convert electrical energy into heat through resistive elements. When electric current flows through these resistive elements, heat is generated, which can then be circulated to warm up a space or water.

Duct heaters, while they can also be electrically powered, are typically integrated into ductwork to provide heat to air that is distributed throughout a building. They are a specific application of electric heating within an existing air distribution system.

Furnaces traditionally refer to systems that may use a variety of fuel sources, including natural gas, propane, or electricity, but they are not exclusively electrical resistive systems. Similarly, boilers are primarily used for heating water or producing steam and may run on different fuel types, including gas or oil, rather than being solely electric resistive.

Thus, the electric heater stands out as the most straightforward example of a heating system that primarily uses electrical resistive elements for operation.