To understand this problem you must take two factors into account:
1. The meridians diverge as you move towards the equator and converge as you move away from it.
2. If you move the same distance north, then south, then east, then west,
your end point will depend upon the relative magnitudes of any convergence or divergence which occur. You will end up at your starting point only if the diveregnce/convergence going north is exactly equal and opposite to the divergence/convergence going south.
This question specifies 600 nm. Each minute of arc on a meridian is one nm, so 600 nm is 10 degrees.
If you are 5 degrees south of the equator and start by going 600 nm (or 10 degrees) north you will get divergence over the first 5 degrees (up to the equator). You will then get the same amount of convergence over the next 5 degrees, as you move away from the equator. So the overall efect of the convergence/divergnce will be zero.
The same will happen during the southerly leg of the journey. So you would end up back where you started.
If you now look at the efects of starting at 85 north or south, you will see that this self-cancelling effect does not occur.