Hi

You need to choose one subject for optional for your UPSC main examination and  there will be two papers for your optional subject.

-) both the optional papers are of 250 marks each
-) your optional papers will be subjective
-) your optional subject question paper will be set in both English and Hindi except for the literature subject
-)  you can choose any one subject of your choice for your optional subject from the the given below list of optional subjects  which include literature subject as well i. e English and Indian languages.

List of optional subject is provided below:-

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

List of literature optional subjects is provided below:

-) English
-) Assamese
-) Bengali
-) Gujarati
-) Hindi
-) Kannada
-) Kashmiri
-) Konkani
-) Malayalam
-) Manipuri
-) Marathi
-) Nepali
-) Odia
-) Punjabi
-) Sanskrit
-) Sindhi
-) Tamil
-) Telugu
-) Urdu
-) Bodo
-) Dogri
-) Maithili
-) Santhali

To help you for further, here's a brief of UPSC examination pattern
UPSC civil services examination  has 3 stages listed below :-

*prelims exam :- it is objective paper and is of qualifying nature , so you just  need to at least qualify it to be eligible for mains examination, the score /marks obtained by you in your prelims examination is not added up or used in your final merit. Your prelims exam consist of 2 papers: -
-) general studies 1
-) general studies II ( csat )
You get 2 hours for each paper

*Mains exam. :- it is descriptive /written examination . There are a total of 9 papers  which are listed below:-
-) paper A :- compulsory indian language.
-) paper B :- english
-) essay
-) general studies I
-) general studies II
-) general studies III
-) general studies IV
-) optional I
-) optional II

The compulsory indian language and english are qualifying in nature . You just need to qualify them. The marks obtained by you in these two papers will not be counted towards merit list.

*Interview :- After qualifying mains, you have to go through interview round, and it is more of test of your personality rather than knowledge.

So, if you managed to sail through all the three stages  then congratulations you can become an ias  officer depending upon  rank obtained by you in UPSC civil services examination ,your preference of services and your category .

For detailed information , I will recommend you to go through our page the link for the same is provided below

https://school.careers360.com/articles/how-to-become-an-ias-officer

If you have any other doubt then feel free to ask them :) we are always here to help you out and guide you

Thank you