A function $\displaystyle {f{{\left({x}\right)}}}$ is one-to-one if every distinct input $\displaystyle {x}$ results in a distinct output $\displaystyle {y}$. This means that no two different values of $\displaystyle {x}$ in the function's domain map to the same value of $\displaystyle {y}$. A practical way to verify if a function is one-to-one is to use the horizontal line test. If any horizontal line crosses the graph more than once, the function is not one-to-one. Check each graph to see if it passes this test.