We are given:
\[
I = \int_0^a \frac{dx}{\sqrt{x}}
\]
This is a standard power rule integral. We can rewrite it as:
\[
I = \int_0^a x^{-\frac{1}{2}} \, dx
\]
The integral of \( x^n \) is \( \frac{x^{n+1}}{n+1} \). Using this formula, we get:
\[
I = \left[ 2x^{\frac{1}{2}} \right]_0^a = 2\sqrt{a} - 0
\]
Thus, the correct answer is \( 2\sqrt{a} \).