I would suggest you to decide first which subject you should take as per your interest, You should follow your interest for it. The degree can be obtained in any stream recognised by University Grants Commission (UGC) to apply for the exam.

UPSC civil services examination needs a complete hold over General Studies papers which are classified under four parts, GS I, GS II, GS III and  GS IV. Each of the GS papers have different subjects within them. Basically, these GS subjects are the papers of Economics, History, Geography, Biology, Physics, Chemistry, Environmental Science, Biodiversity, etc. 

Good Luck!