A Measurement is defined as_.

a. system of prefixes
b. the distance between two points
c. an exact quantity people agree to use for comparison
d. the interval between two events

c is fancy language, but it is almost correct, it is the best answer.

Measurement don't have to be exact, and usually are not...they have some precision, but nothing more than that. A pinch of salt is a measurement, but it is not exact.

an exact quantity people use for comparison

c. an exact quantity people agree to use for comparison

The answer is (c) an exact quantity people agree to use for comparison.

Explanation:
A measurement is a process of determining the quantity or amount of something. It involves comparing the unknown quantity with a known quantity, which serves as a standard or a reference. This known quantity is what we call a measurement.

For example, when we measure the length of an object using a ruler, the markings on the ruler represent specific units (such as inches or centimeters) that have been agreed upon as standards for measuring length. These units act as the known quantities against which we compare the length of the object. The result of this comparison helps us determine the measurement or the exact quantity of the object's length.

Therefore, a measurement is an exact quantity that people agree to use as a reference for comparing and quantifying various characteristics or properties of objects, events, or phenomena. It provides a standardized way of expressing and communicating the quantity being measured.