Which of the following statements is always true for a function?

Correct answer: Every input has exactly one output

For a function, every element of the domain (input) must be associated with exactly one element of the codomain (output), so statement A is always true. Statements B and C describe additional one-to-one behavior, which is not required for every function; different inputs may have the same output. Statement D is false because no input in the domain can be left without an associated output. Exam tip: To test a mapping, check that exactly one arrow starts from each input.