I have read many other solutions but this situation seems unique.
\documentclass{exam}
\usepackage{multicol}
\usepackage{amsmath}
\begin{document}
\begin{questions}
\question Simplify the following:
\begin{multicols}{4}
\begin{parts}
\part $8^\frac{2}{3}$
\part $\dfrac{16^{\frac{2}{3}}}{16^{\frac{1}{6}}}$
\part $9^\frac{1}{3}\cdot 9^{-\frac{5}{6}}$
\part $\left(25^{\frac{1}{3}}\right)^{\frac{3}{2}}$
\end{parts}
\end{multicols}
\end{questions}
\end{document}

