B. When you find the root of 38, you will get 6.164. So that eliminates A and D. Then when you look closer, you will see that C looks closer to 6.4 or even 6.5 than 6.164, which leave you with B being the answer.
The domain of the function is \[A \ge0\] If A is negative we get an imaginary number for the radius. This makes sense because the area of a circle cannot be negative.
