Which function is classified as a rational function?

A rational function is defined as a function that can be expressed as the ratio of two polynomial functions. This means it takes the form of one polynomial p(x) divided by another polynomial q(x), where q(x) is not equal to zero.

The choice that exemplifies this definition is the one presented as a fraction of p(x) over q(x). In this case, both the numerator and the denominator must be polynomials for the function to be classified as rational. This differentiates it from the other options, which represent different types of functions.

The first choice represents a quadratic function, which is a specific type of polynomial, but not in the structure of a quotient. The third choice outlines a linear function, which is also a polynomial but does not exhibit the ratio aspect. The fourth option is a constant function, which likewise does not align with the classification as a rational function.

Thus, the option that fits the definition of a rational function due to its structure of being a quotient of two polynomials is the correct answer.