Answer
The horizontal-line test can be used to determine if a function is a one-to-one function because if every horizontal line intersects the function exactly once, then for every $y$ value of the function, there is a unique $x$ value.
Work Step by Step
The horizontal-line test can be used to determine if a function is a one-to-one function because if every horizontal line intersects the function exactly once, then for every $y$ value of the function, there is a unique $x$ value.