Lumen is a unit used to measure luminous flux, which represents the total amount of visible light emitted by a light source per unit of time. The correct answer is [A] luminous flux. Luminous flux quantifies the brightness of light as perceived by the human eye, regardless of the direction in which the light is emitted. It is crucial in determining the light output of light bulbs, LED lamps, and other light sources, providing a standardized measure of their brightness. Luminous intensity (option B) measures the brightness of a light source in a specific direction and is expressed in candelas (cd). Illuminance (option C) measures the amount of light incident on a surface per unit area and is expressed in lux (lx). Brightness (option D) is a subjective term referring to the perceived intensity of light, which can vary based on individual perception and environmental factors.