As per your request I am here giving you list of optional subjects for CS (Main) exam of Union Public Service Commission (UPSC).

Scheme - CS (Main)
Paper-I
One of the Indian Languages to be selected by the candidate from the 18 languages included in the VIIIth Schedule to the Constitution (Qualifying Paper)
300 Marks

Paper-II English (Qualifying Paper)
300 Marks

Paper-III
Essay
200 Marks

Papers IV & V
General Studies (300 Marks for each paper)
600 Marks

Papers VI, VII, VIII & IX
Any two subjects (each having 2 papers) to be selected from the prescribed optional subjects (300 marks for each paper)
1200 Marks

List of Optional Subjects

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

List of Optional Subjects (Literature):

Arabic
Assamese
Bengali
Chinese
English
French
German
Gujarati
Hindi
Kannada
Kashmiri
Konkani
Marathi
Malayalam
Manipuri
Nepali
Oriya
Pali
Persian
Punjabi
Russian
Sanskrit
Sindhi
Tamil
Telugu
Urdu