#### Answer

D. $-\sqrt 6$

#### Work Step by Step

As shown on page 445, we know that $a^{\frac{m}{n}}=\sqrt[n] a^{m}=(\sqrt[n] a)^{m}$.
Therefore, $-6^{\frac{2}{4}}=-(6^{\frac{2}{4}})=-(6^{\frac{2\div2}{4\div2}})=-(6^{\frac{1}{2}})=-(\sqrt[2] 6^{1})=-(\sqrt 6)=-\sqrt 6$.