The answer to your qn would definitely be C.
And this is because..
When you have M and N..they're connected in parallel. Lamps form resistence..Hence, their overall resistance is less than both, right?
However, when you are left with one lamp, in series with the ammeter.. The resistance is increased.
Thus, the current, is decreased..
If you analyse the circuit, you will see that this proves to be right!
Hope I helped