The correct answer is: C. Lumen
Lumen (lm) is the SI unit of luminous flux, a measure of the total quantity of visible light emitted by a source. It is defined as the luminous flux emitted by a point source of light of one candela uniformly in all directions.
Lux (lx) is the SI unit of illuminance, a measure of the amount of light that is incident on a surface. It is defined as the luminous flux per unit area, and is equal to one lumen per square meter.
Candela (cd) is the SI unit of luminous intensity, a measure of the brightness of a light source. It is defined as the luminous intensity of a point source of light that emits monochromatic light of frequency 555 nanometers and has a radiant intensity of 1/683 watt per steradian.
Lumen second (lm·s) is a unit of luminous energy, and is equal to the luminous flux emitted by a source of one lumen in one second. It is not an SI unit.