#### Answer

C. A math sentence that defines the relationship between quantities.

#### Work Step by Step

Recall, a formula allows us to relate one quantity to one or more other quantities. For instance, in the equation, $A=1/2 \times b \times h$, the area of a triangle is related to two other quantities—the length of the base of the triangle and the height of the triangle. Thus, the answer "a math sentence that defines the relationship between quantities" is correct.