hello abhishek!!

Its good to hear that You want to BE AN IFS OFFICER.

If you are from humanities background and if you want to be an IFS officer which involves handling diplomatic relations with foreign countries it would be better you take POLITICAL SCIENCE  as your optional as it invovles both parts of polity and international affairs in it.Please see the syllabus of it ,go through the previous papers of the subject and if you can get notes or material of any institute please read it and read ncert of political science and see whether you are able to enjoy and understand and able to understand the question asked in upsc and analyse.Then only go for it.

however, as it is for UPSC, I would say any optional would be fine to take as every optional is same as other and each of it has got its own benefits and problems.

all the very best 

thank you