Always verify the final square root before selecting the option.
Here,
\[
9^2+12^2=81+144=225
\]
and
\[
\sqrt{225}=15,
\]
not 13. Also, \((9,12,15)\) is a multiple of the famous Pythagorean triplet \((3,4,5)\).
Hence, the correct option is (D) 15 km, North-East.