In mathematics, a function is a special type of relation between two sets: the domain (input) and the codomain (output). By definition, a function assigns each element in the domain to exactly one element in the codomain. This is known as the "uniqueness" property of a function. It is one of the key characteristics that distinguishes functions from general relations, where a single input could relate to multiple outputs.
Given this definition, the correct answer to the question "A function is a relation in which each element in the input is mapped to how many elements in the output?" is:
exactly one.
This ensures that for every input value there is a single, well-defined output value, making the function predictable and consistent across its entire domain.
