smc

Because ${\log }_7({\log }_3({\log }_{2}x))=0$, we deduce ${\log }_3({\log }_{2}x)=1$, and thus ${\log }_{2}x=3$. Therefore, $x=8$, which means $x^{-1/2}=\frac{1}{2\sqrt{2}}$. Since this does not match any of the answer choices, the answer is $\boxed{\text{none of these}}$.