Candela: The Unit of Luminous Intensity

The question asks to identify what physical quantity Candela is a unit of. Candela ($\text{cd}$) is a fundamental unit in the International System of Units (SI). It is one of the seven SI base units.

Let's examine each option to understand what they measure and their respective units:

• Luminous intensity: Candela is the SI base unit specifically defined for luminous intensity. Luminous intensity measures the amount of light emitted by a light source in a particular direction. It is a measure of the power emitted by a light source in a specific direction, weighted by the luminosity function (a standardized model of the sensitivity of the human eye to different wavelengths of light). A common example helps understand luminous intensity: a regular candle emits light with a luminous intensity of approximately one candela in a given direction.
• Luminous Flux: Luminous flux is the total amount of light emitted by a source in all directions, as perceived by the human eye. While related to luminous intensity, it represents the total light output. The SI unit for luminous flux is the lumen ($\text{lm}$). One lumen is defined as the luminous flux produced by a point source of one candela across a solid angle of one steradian ($\text{1 lm = 1 cd} \cdot \text{sr}$).
• Wavelength: Wavelength is a property of a wave and describes the spatial period of a periodic wave. It is the distance over which the wave's shape repeats. For light, wavelength determines its color. The SI unit for wavelength is the meter ($\text{m}$), or often nanometers ($\text{nm}$) for visible light, since light wavelengths are very small.
• Frequency: Frequency is another property of a wave, indicating how many cycles of a wave pass a given point per unit of time. For light, frequency is related to its energy. The SI unit for frequency is the hertz ($\text{Hz}$), which is equivalent to one cycle per second ($\text{s}^{-1}$).

Based on these definitions, it is clear that Candela is the standard unit used to measure luminous intensity.