Answer
$r^{\frac{3}{4}}$, or (D)
Work Step by Step
When dividing exponents, keep in mind that $\frac{x^a}{x^b} = x^{a-b}$
Applying that rule to this problem: $\frac{r^{\frac{1}{2}}}{r^{-\frac{1}{4}}} = r^{\frac{1}{2}-(-\frac{1}{4})} = r^{\frac{3}{4}}$
