What is defined as a ratio that compares two quantities?

The correct choice is a rate because it specifically describes a comparison between two different quantities, typically involving time or ratio. For example, when calculating miles per hour, you are comparing distance (miles) with time (hours), which illustrates the concept of a rate.

A rate encompasses the relationship of two distinct but related quantities, allowing for a clear understanding of how one quantity changes in relation to another. While proportion and factor are related concepts in mathematics, they do not convey the same direct comparison of two quantities under specific contexts that a rate does. Proportion typically involves an equation that states that two ratios are equal, while factor refers to a quantity that when multiplied with another produces a given number. Therefore, rate is the most precise definition for a ratio that compares two quantities.