2014 AMC 12A Problem 7

Writing the terms as powers of $3,$ they are $3^{\frac{1}{2}},$ $3^{\frac{1}{3}},$ $3^{\frac{1}{6}}.$ The common ratio is $\dfrac{3^{\frac{1}{3}}}{3^{\frac{1}{2}}}=3^{-\frac{1}{6}}.$ The fourth term is $3^{\frac{1}{6}}\cdot3^{-\frac{1}{6}}=3^{0}=1.$ Thus, the correct answer is A.