Which of the following defines a function in mathematics?

A function in mathematics is defined as a relation in which each input (often referred to as the independent variable) corresponds to exactly one output (the dependent variable). This means that for every value you plug into the function, there is a single, unique result. This unique pairing is crucial because it ensures that the function behaves predictably; if an input were to produce multiple outputs, it would violate the definition of a function.

The concept of a function can be visualized with graphs, where the vertical line test is often used—if any vertical line crosses the graph at more than one point, the relation is not a function. Therefore, the correct answer highlights the essential characteristic of a function in mathematics, distinguishing it from other types of relations or sets.