Following is an excerpt from the document from FPSC named 'Rules For Competitive Exam (CSS)', which means you will have to select two subjects of marks 100 each, or one subject of marks 200.
Group-II: (One subject carrying 200 marks from code 16-17 OR
two subjects carrying 100 marks each from code 18-21 can be opted).
Here's the hyperlink to that document. http://www.fpsc.gov.pk/icms/admin/we...2017-Rules.pdf