Step 1: Find points of intersection.
Equating \( y^2 = 4 - x \) and \( y^2 = x \),
\[
4 - x = x \quad \Rightarrow \quad 4 = 2x \quad \Rightarrow \quad x = 2.
\]
So, the region extends from \( x = 0 \) to \( x = 2 \).
Step 2: Compute area using integration.
\[
A = \int_0^2 \left( \sqrt{4-x} - \sqrt{x} \right) dx.
\]
Solving the integral, we get:
\[
A = \frac{16\sqrt{2}}{3}.
\]
Step 3: Selecting the correct option.
Since \( \frac{16\sqrt{2}}{3} \) matches, the correct answer is (D).