To find the area of a circle, you can use the formula:
\[ A = \pi r^2 \]
where \( A \) is the area and \( r \) is the radius of the circle.
Given that the diameter is 21 mi, the radius \( r \) is half of the diameter:
\[ r = \frac{21}{2} = 10.5 \text{ mi} \]
Now substituting \( r \) into the area formula:
\[ A = 3.14 \times (10.5)^2 \]
Calculating \( (10.5)^2 \):
\[ (10.5)^2 = 110.25 \]
Now substituting back into the area formula:
\[ A = 3.14 \times 110.25 \]
Calculating this:
\[ A = 346.185 \]
Rounding to the nearest hundredths place:
\[ A \approx 346.19 \text{ mi}^2 \]
Therefore, the correct response is:
346.19 mi²