Final answer:
Illuminance is the luminous flux per unit area incident on a surface, measured in lux. It's a critical measure for determining how much light is available to illuminate a given area.
Step-by-step explanation:
The best definition of illuminance is A. luminous flux density incident on an object per unit area. Illuminance measures how much luminous flux (light) falls on a surface area and is given in units of lux (lx). To further understand the concept, consider a light bulb emitting light in all directions. The illuminance is then the amount of light per square meter that lands on a surface at a given distance from the bulb. This is crucial in practical applications where we need to know how much light illuminates a surface for activities like reading or working.