Which of the following expressions represents a function?
x = 1
{(3, 2), (3, −2), (4, 5), (4, −5)}
y = 4x − 1
4x2 + y2 = 16

A function can have one x-value for each y-value Or even, two difference x-values for one y-value.

And out of the four options, only A and C represent a function. The other two options have the same x value for two y-values.

One way to test if it is a function is by the vertical line test.