Final answer:
A number must be part of a quantitative observation, providing a measurable quantity with a specific unit.
Step-by-step explanation:
When making a quantitative observation, it is essential that the observation includes a number to indicate an amount and a unit to define the scale of measurement. For example, stating that 'the melting point of crystalline sulfur is 115.21° Celsius' is a quantitative observation because it provides both a number (115.21) and a unit (° Celsius) to express the quantity measured. Therefore, among the options provided, a number must be part of a quantitative observation.