Step 1: Approach
Use geometry instead of calculus.
Step 2: Shape
Squaring $y=\sqrt{16-x^2}$ gives $x^2+y^2=16$, a circle of radius 4. With $y\ge0$ and $0\le x\le4$ the region is exactly one quarter of the circle.
Step 3: Area
\[ \frac14\pi(4)^2=4\pi \]
Option (D).
Final Answer:
The region is a quarter of a circle of radius 4, so its area is 4 pi, option (D).
\[ \boxed{4\pi} \]