Chetan, there is no stream specialisation for appearing for UPSC which is the entrance examination for IAS positions.

UPSC is a 3 stage  selection process involving Prelims, Mains and Interview which tests your personality test. Prelims has 2 Papers namely General Studies and CSAT.

General Studies has Current Events, History of India and Indian National Movement, Indian and World Geography, Indian Polity and Governance, Economic and Social Development, Environmental Ecology, Biodiversity and Climate Change and General Science.

CSAT has Comprehension, Interpersonal skills including communication skills, Logical Reasoning and Analytical ability, Decision making and problem solving, General Mental Ability, Basic numeracy and Data Interpretation.

If you look at the prelims subjects, it is general subjects but when it comes to selecting optional subjects your Bachelor's degree will come in hand. Statistics show that Candidates who have taken subjects like Public Administration, Geography, Anthropology, History, General Science have scored better than counterparts. So if you can study any of these subjects in your BA, they will  help you in your preparation in the long run.