To find the value of $\sqrt{-36}$, we need to use the concept of imaginary numbers. The square root of a negative number can be expressed in terms of $i$, where $i$ is the imaginary unit defined as $\sqrt{-1}$.

So, we can rewrite $\sqrt{-36}$ as follows:

$\sqrt{-36} = \sqrt{36 \cdot (-1)} = \sqrt{36} \cdot \sqrt{-1} = 6i$

Thus, the expression $\sqrt{-36}$ is equivalent to $6i$.

The correct answer is:

$\boxed{\text{C. } 6i}$

Do you want details or have any questions? Here are five more questions you might ask:

1. How do you simplify the square root of a negative number using imaginary units?
2. What are the properties of imaginary numbers?
3. How do you multiply and divide complex numbers?
4. What is the significance of the imaginary unit $i$ in mathematics?
5. How do you convert a complex number from rectangular form to polar form?

Tip: Remember that $\sqrt{-1} = i$ is the fundamental definition of the imaginary unit. This concept allows us to work with square roots of negative numbers in the complex number system.