A linear function is one where if you wrote an equation for it, the variables
in the equation would all be to the first power ... no squares or higher powers.
With that kind of equation, the graph of the function is a straight line.
A). Side length and area of a square
Area of a square = (side length)² .
The side length is squared. The function is not linear.
B). Radius and area of a circle
Area of a circle = (π) times (radius)²
The radius is squared. The function is not linear.
C). Diameter and circumference of a circle
Circumference = (π) times (diameter)
No squares or other higher powers. The function is linear.
D). Side length and surface area of a cube
Surface area of a cube = 6 (side length)²
The side length is squared. The function is not linear.