From the graph of $f(x) = x^3 + x$, we can apply the horizontal line test.

A function is one-to-one if every horizontal line crosses the graph at most once. In this case, for any horizontal line, the graph crosses at most once. Therefore, the function $f(x) = x^3 + x$ is one-to-one.

Thus, the answer to the question is: "y".

Do you have any questions or need further clarification?

Related Questions:

1. What is the mathematical definition of a one-to-one function?
2. How does the derivative of a function help in determining if it is one-to-one?
3. Can every cubic function be one-to-one?
4. How would the graph of $f(x) = x^3 - x$ differ from $f(x) = x^3 + x$?
5. How can we algebraically determine if a function is one-to-one?

Tip:

When graphing, remember that the horizontal line test is a quick visual method to check if a function is one-to-one.