The definition of a function is that for each input, there is only one output.
Thus, the answer to your question is A, as it is the only answer option that each input has only one output.
In answer choice B, the input -1 has outputs of both -2 and -5.
In answer choice C, the input 4 has outputs of both 3 and 0.
Finally, in answer choice D, the input -4 has outputs of both -4 and 4.
Therefore, answer A is the only function because it is the only option that follows the definition.
Hope this helps! :)
