Which of the following light sources is the least efficient?

Incandescent light sources are known for their low energy efficiency compared to other types of lighting. They work by heating a metal filament until it glows, which results in a significant amount of energy being wasted as heat rather than being converted into visible light. Typically, incandescent bulbs convert only about 10% of the energy they consume into light, with the remainder lost as heat.

In contrast, options like high-pressure sodium lamps, low-pressure sodium lamps, and fluorescent lights utilize more advanced methods of producing light, resulting in better energy efficiency and lower operating costs. Fluorescent lights, for instance, can convert a much larger percentage of electricity into light and have a longer lifespan. High-pressure sodium lamps are also more efficient than incandescent bulbs, providing a higher output of light per watt consumed.

Therefore, incandescent lighting stands out as the least efficient choice due to its significant energy waste, making it less desirable for applications where energy savings and longevity are important.