Which equation does not represent a function?

Question

1. y = 4
2. y = x2 minus 4
3. y = x minus 4
4. x2 + y2 = 4

Answer 1

4.

x^2 + y^2 = 4 is a circle of radius 2.

A circle does not pass the vertical line test.

Answer 2

4. x^2 + y^2 = 4
does not represent a function

Answer 3

The last one as for a given x<2 you get two values of y

Answer 4

#4 is not a function
Definition of a function - for every 'x', there is one and only one 'y'. #4 isn't a function because for many values of x, there are two values of y