The expression you need to simplify is $\sqrt{-25} - 4$.

Let's break this down:

1. The square root of a negative number involves imaginary numbers. In this case, $\sqrt{-25} = 5i$, where $i$ is the imaginary unit, $i = \sqrt{-1}$.

2. Now, simplify the expression: $\sqrt{-25} - 4 = 5i - 4$

The correct answer is $5i - 4$, which corresponds to option C.

Would you like more details or explanations?

Here are 5 related questions:

1. What is the square root of negative numbers called in complex number theory?
2. How would you simplify $\sqrt{-9} - 3$?
3. What is the value of $i^2$ in imaginary numbers?
4. How do you add and subtract complex numbers?
5. Can a negative number have a real square root?

Tip: Always remember that the square root of a negative number involves the imaginary unit $i$.