Which of the following is NOT one of the main types of electric heating systems?

Solar heating systems are primarily associated with utilizing sunlight to generate heat, typically through solar thermal collectors. This technology harnesses solar energy and is not categorized as an electric heating system, which directly uses electricity to produce heat. In contrast, electric heat pumps, electric radiant heating, and resistance heating all involve electricity in their heating processes.

Electric heat pumps operate by transferring heat from one place to another using electrical energy, making them a prominent form of electric heating. Electric radiant heating systems provide warmth through electrically heated surfaces, such as floors or ceilings. Resistance heating, on the other hand, involves converting electrical energy directly into heat through resistive materials.

Therefore, solar heating systems do not belong to the category of electric heating systems, confirming that the answer provided is indeed correct.