Rational functions need to be a fraction and need have to a polynomial in the numerator and a polynomial in the denominator.
Choice A is not a fraction. Nope.
Choice B has the 2 in the denominator, but that's not a polynomial. Nope.
Choice C is not a fraction. Nope.
Choice D has x² in its denominator which is a polynomial in disguise - x² + 0x = 0. The numerator has x-3. Bingo.
Thus choice D is best.
