Nice to know that you want to be an IAS and wish you Best of luck.

Now regarding your question, I would say there is no such subject which can ensure your chances more. See UPSC is an examination which requires a special attention that is to get a job through UPSC you need to study specifically as per the UPSC syllabus. Thus there are no options available that can ensure you more about the selection. I would recommend you that you should take the subject you like the most. This is a very important point because if you take the subject you like then while studying it you will not feel bored or get exhausted. This will help you to study your BSc subject as well as your preparation for UPSC. So take the subject you like the most.

Now you can take some subjects which are in your UPSC syllabus like BSc Economics. So its better for you to go for the subject you like so that you can focus more on your preparation and don't get frustrated due to your BSc courses. But I assure you that the things you will learn in BSc will not be needed exactly in your UPSC preparation. Instead you will find that foundational knowledge to the subject is important. This is the reason why I stated that you should pursue the subject you like and keep a focused plan for your UPSC preparation.

To know more about UPSC visit the link below:

https://competition.careers360.com/exams/upsc-civil-services-exam

I hope my answer helps. All the very best for your future endeavors!