In a function, each input must have only one output. Outputs can have multiple inputs, but each input must have one output. For example, {(4,-1), (4,-2)} is not a function, as 4 has two outputs.
In B, -2 has two outputs, 3 and 1, so it is not the answer.
In C, 3 has two outputs, 5 and 2, so it is not a function.
In D,-1 has two outputs, 1 and 5, so it will not work.
In A, each number has one output, so it is a function.
