Answer:
D: 10 pi km
Explanation:
That triangle is a right angled triangle. This means you can use pythagoras' theorem on it.
a^2+b^2=c^2
Here we have 6^2+8^2 = 36 + 64 = 100 = 10^2 so the diagonal is 10km.
The diagonal is also the diameter of the circle. The circumference of a circle is pi * the diameter, so the answer is 10 pi km: d.