What type of number is used to represent a quantity less than zero?

The type of number used to represent a quantity less than zero is classified as a negative number. Negative numbers are essential for modeling values that fall below zero, such as temperatures in a cold climate or financial debts. They extend the number line in the opposite direction from positive integers, allowing for a comprehensive numerical system that accommodates both sides of zero.

Positive numbers represent quantities greater than zero; whole numbers are non-negative integers that include zero and positive integers; and integers encompass both positive and negative numbers as well as zero. Therefore, within this context, negative numbers specifically refer to those values that indicate quantities less than zero, making them the correct choice.