Hello Nishi!
UPSC Examination is mostly about testing your General Knowledge, General Awareness/Current Affairs and your language skills.
Hence here is a list of those subjects of humanity which would eventually be helpful for your UPSC preparation:

• Anthropology
• Economics
• Geography
• Geology
• Indian History (General Knowledge questions for UPSC are mostly asked from the Indian History)
• Law (B.A. LLB)
• Philosophy
• Political Science and International Relations (Helpful for general Knowledge questions in UPSC)
• Psychology
• Sociology

Law, Psychology and Economics are the subjects which are not only useful for UPCS, but these subjects have various job opportunities and a lot of scope in future too. While history and political science would be helpful for improving your General Knowledge, which forms a very important part of UPSC.

Hence be flexible and decide accordingly. Eventually what matters is your interest in the subject which you would prefer to opt for B.A.and UPSC.

All the best! Do well! :)