Hello aspirant,
Here are UPSC optional subject list...
Vetinary Science
Anthropology
Botany
Chemistry
Civil Engineering
Commerce and Accountancy
Economics
Electrical Engineering
Geography
Geology
History
Law
Management
Mathematics
Mechanical Engineering
Medical Science
Philosophy
Physics
Political Science and International Relations
Psychology
Public Administration
Sociology
Statistics
Zoology
Literature of anyone the following language: Assamese, Bengali, Bodo, Dogri, Gujarati, Hindi, Kannada, Kashmiri, Konkani, Maithili, Malayalam, Manipuri, Marathi, Nepali, Oriya, Punjabi, Sanskrit, Santhali, Sindhi, Tamil, Telugu, Urdu, English.
Agriculture
Good luck.

Hello Nitish,

The optional subjects for UPSC are;

Group 1:

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

Also there are a bunch of subjects (group 2) based on languages Viz;Assamese, Bengali, Bodo, Dogri, Gujarati, Hindi, Kannada, Kashmiri, Konkani, Maithili, Malayalam, Manipuri, Marathi, Nepali, Oriya, Punjabi, Sanskrit, Santhali, Sindhi, Tamil, Telugu, Urdu, English.

in my opinion  picking a  subject which you  feel  comfortable with and you can cope up for long period of time would be a best option and a good motivation for you to carry on . Worrying about language medium is  not an issue because almost all the subjects of group 1  contain available in Hindi medium as well. Also it would be advisable to start polishing your English because of force is students with English have more content available and have an edge over the others. 

I hope this helps.