Hello, 

Let me tell you what are all the subjects that one can choose in Mains exam,

• Mechanical Engineering,
• Medical Science,
• Philosophy,
• Physics,
• Political Science and International Relations,
• Psychology,
• Public Administration,
• Sociology,
• Statistics,
• Zoology
• Agriculture,
• Animal Husbandry and Veterinary Science,
• Botany,
• Chemistry,
• Civil Engineering,
• Commerce and Accountancy/Economics,
• Electrical Engineering,
• Geography,
• Geology,
• History,
• Law,
• Management,
• Mathematics

You can choose any of the above and prepare for it for the sake of mains exam. There are basically two options. You can go for doing bachelor's degree in any of the above subjects or choose your own stream (if you are interested in aviation) and then go for writing mains exam. It totally depends upon your desire. If you are only interested in civil services then I would suggest you to go for bachelor's degree in above subjects. If you are really interested in taking up aviation then it's your choice. Although the eligibility criteria for writing upsc is only to have a bachelor's degree and you have to prepare for the subject you have chosen in Mains separately if you didn't do them in your bachelors. The other side of the story is degree in aviation has a glorious career and plenty of opportunities and that might help if you change your mind about becoming ias(only if you are interested in aviation) . Otherwise if your only goal is upsc then any of the degree is quite ok.Choice is yours. Hope you may find this helpful. 

All the best ......