A function `f(x)` is one-to-one if every distinct input `x` results in a distinct output `y`. This means that no two different values of `x` in the function's domain map to the same value of `y`. A practical way to verify if a function is one-to-one is to use the horizontal line test. If any horizontal line crosses the graph more than once, the function is not one-to-one. Check each graph to see if it passes this test.

Graphing window shows horizontal axis: -5 to 5, vertical axis: -5 to 5. Start Graph, Color blue

x	y
-4	-10
-3	-8
-2	-6
-1	-4
0	-2
1	-0
2	2
3	4
4	6
5	8

Graphing window shows horizontal axis: -5 to 5, vertical axis: -5 to 5. Start Graph, Color blue

x	y
-5	-1.71
-4	-1.587
-3	-1.442
-2	-1.26
-1	-1
0	0
1	1
2	1.26
3	1.442
4	1.587
5	1.71