The correct answer to the given question is: A) $\sqrt{x}$.

Let me know if you need further details or clarifications!

Here are 5 related questions:

1. What is the inverse of a square root function?
2. How do you find the derivative of $\sqrt{x}$?
3. What is the domain of the function $f(x) = \sqrt{x}$?
4. Can $a$ be negative in this scenario? Why or why not?
5. How would the solution change if the equation was $a^2 = x$?

Tip: When solving for variables involving square roots, remember to account for both positive and negative roots unless the domain specifies otherwise.