Question

Which relation is a function?

Answer 1

let's recall the vertical line test, if a vertical line hits the graph or points twice, then is NOT a function.  Check the picture below.

Answer 2

A very easy way to find out if a graph is a function is to use the vertical line test.

Look at each graph and mentally draw a vertical line that goes through each of its points.

If that line goes through more than one point (2, 3, 4...) then that graph is not a function.

That's because a function, is an equation for which each input (x value) has exactly one output (y value). There cannot be two points of the same x value, with two different y values.