The correct option is: (D) 300√61 km
Let the point A represent the airport.
Let the plane-I fly towards North.
.'. Distance of the plane-I from the airport after 1 1/2 hours
Let the plane-II fly towards West.
.'. Distance of the plane-II from the airport after 1 1/2 hours
Now, in right ΔABC, using Pythagoras theorem, we have
Thus, after 1 1/2 hours, the two planes will be 300√61 km apart from each other.