We can solve this problem using the formula:

distance = speed x time

Let`s assume that the two trains start at time t=0.

After t hours, the distance traveled by the northbound train is 60t km.

Similarly, the distance traveled by the southbound train is 40t km.

The distance between them is the sum of these distances, which is:

distance = 60t + 40t = 100t km

We want the distance to be 150 km. So we set up the equation:

100t = 150

Solving for t, we get:

t = 1.5 hours

Therefore, the two trains will be 150 km apart after 1.5 hours.

So, the answer is option (a) 3/2.