Which of the following is an example of the axiom of trichotomy?

• A: $x \neq y$. This statement means that $x$ is either less than $y$ or greater than $y$ ($x < y$ or $x > y$). It is not one of the three distinct relations specified by the axiom, but rather a combination of two of them.
• B: $y < x$. This is equivalent to $x > y$, which is one of the three fundamental relations in the axiom.
• C: $x < y$. This is one of the three fundamental relations in the axiom.
• D: $x = y$. This is one of the three fundamental relations in the axiom.

Options B, C, and D are all individual statements that are part of the axiom of trichotomy. Since the question asks for "which one of the following", and multiple options are technically correct parts of the axiom, we will select one of them. Option C, $x < y$, is a direct statement of one of the three conditions.

The correct option is C.

C. $\boxed{x < y}$