Phrase C, '2 o'clock in the afternoon', describes an unknown or changeable quantity because it represents a time that can vary throughout the day.
An unknown or changeable quantity is described as a variable.
It represents a value that can change or is not yet known.
Out of the given phrases, phrases A, B, and D are known quantities or measurements because they provide specific values or measurements.
Phrase C, however, represents a time which can vary throughout the day and is considered an unknown or changeable quantity.
For example, at 2 o'clock in the afternoon, the position of the hands on a clock is different each day.