#### Answer

not a real number

#### Work Step by Step

$\sqrt[4] -16=\sqrt[4] (-1\times16)=\sqrt[4] -1\times\sqrt[4] 16$
We cannot simplify any more, as $\sqrt[4] -1$ is not a real number. We cannot find a number $a$ such that $a^{4}=-1$. In general, we cannot take the root of a negative number when the index (the number above the radical) is an even number.