A function `f(x)` is one-to-one if every distinct input `x` results in a distinct output `y`. This means that no two different values of `x` in the function's domain map to the same value of `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.