Think abt political science also as it will cover 500 marks of ur optionals in css as such,

political science 200

public administration 100

constitutional law 100

international law 100

total 500
All the three papers mentioned are the sub-subjects of political science wich u hv 2 study in masters of pol.sc. and syllabus is nearly the same.
and opt one subject more scoring 100. I have opted for pashto. I also did pre-medical and found political science quite easy and interesting. Decision depends upon ur own interest. Best of luck