**The correct option is: (D) 300√61 km**

**Explanation:**

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.