Answer
Student D is correct, as multiplying $m, n$ by constants yields different values, so the function is not well-defined.
Work Step by Step
Consider the pairs of integers $m = 4, n = 3$ and $m = 8, n = 6$. From the definition, the first pair yields $\frac{16}{3}$ as the result, while the second pair yield $\frac{64}{6} = \frac{32}{3}$ as the result. As these are not equal, this means $h(\frac{m}{n})$ is not well defined.