The question is asking for the square root of $-16$.

To solve this, recall that the square root of a negative number involves imaginary numbers. The square root of $-1$ is represented by the imaginary unit $i$, where:

$i = \sqrt{-1}$

For $-16$, we break it down as follows:

$\sqrt{-16} = \sqrt{16} \times \sqrt{-1} = 4i$

Thus, the square root of $-16$ is $4i$.

The correct answer is $4i$ (the last option).

Would you like further details or have any questions?

Here are five related questions for further practice:

1. What is the square root of $-25$?
2. How do you simplify $\sqrt{-9}$?
3. What is the value of $i^2$?
4. How do you multiply two complex numbers involving $i$?
5. What is the meaning of an imaginary number?

Tip: When dealing with square roots of negative numbers, always remember that they involve imaginary numbers, represented by $i$.