## Core Concept
The SI unit of brightness of a point light source is related to the measurement of light intensity. Brightness, in this context, refers to the amount of light emitted by a source per unit solid angle. The relevant physical quantity is **luminous intensity**, which characterizes the brightness of a light source.

## Why the Correct Answer is Right
The correct unit for luminous intensity, which is a measure of the brightness of a point light source, is the **candela (cd)**. The candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 × 10^12 Hz and that has a luminous intensity of 1/683 watt per steradian. This definition makes the candela a fundamental unit in the International System of Units (SI) for measuring the brightness of a point light source.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is blank and does not provide a unit for consideration.
- **Option B:** This option is also blank and does not offer a valid unit.
- **Option C:** Similarly, this option is blank and does not present a viable choice.
- **Option D:** This option is blank as well, failing to provide a recognizable unit for brightness.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **candela (cd)** is not only the unit of luminous intensity but also a base unit of the SI system, emphasizing its fundamental role in quantifying light sources' brightness. This is particularly relevant in fields such as ophthalmology, lighting engineering, and physics.

## Correct Answer Line
**Correct Answer: C. candela.**