Which phase describe a nonlinear function?
a. the area of a circle as a function of the radius
b. the perimeter of a square as a function of the side length
c the cost of gasoline as a function of the number or gallons purchased
d the distance traveled by a car moving at constant speed as a function of time

The correct answer would be choice A).

When the radius of a circle increases, the area does not increase linearly; because the equation for the area is πr², when the radius goes up, the area goes up not by a constant, but by r².

Choices B), C), and D) all describe linear functions; as the side length of a square, the number of gallons of gas purchased, or the time driving increase, the perimeter of the square, the cost of gasoline, and the distance traveled by the car increase at a constant rate.