B

Step 1: Understand the definition of a lux.
The lux (symbol: lm/m²) is a unit of illuminance, which measures the amount of light that falls on a surface per unit area. It is defined as one lumen per square meter.
Step 2: Differentiate between the options.
- Option A (Luminous intensity): Measured in candelas, not lux.
- Option B (Illuminance): Correct, as lux defines illuminance.
- Option C (Transmission coefficient): This refers to the fraction of incident light that passes through a surface, not measured in lux.
- Option D (Luminous efficiency): Measured in lumens per watt, unrelated to lux.
Therefore, the correct option is B.