To find the area of a circle, you can use the formula:
\[ \text{Area} = \pi r^2 \]
where \( r \) is the radius of the circle. The radius is half of the diameter.
Given that the diameter is 21 mi, the radius \( r \) is:
\[ r = \frac{21}{2} = 10.5 \text{ mi} \]
Now, we can substitute the radius into the area formula:
\[ \text{Area} = 3.14 \times (10.5)^2 \]
Calculating \( (10.5)^2 \):
\[ (10.5)^2 = 110.25 \]
Now plug that back into the area formula:
\[ \text{Area} = 3.14 \times 110.25 \approx 346.19 \text{ mi}^2 \]
Rounding to the nearest hundredths place gives:
\[ \text{Area} \approx 346.19 \text{ mi}^2 \]
Therefore, the correct response is 346.19 mi squared.