Hello,

Optional paper is only descriptive paper where you need to write answers for the provided questions, it's syllabus is equivalent to graduation studies, hence what most aspirants do is they go for their graduation subject such as someone with BA Hons in Political Science can go for it as optional however there are exceptions also like last year one of the topper from IIT background, she chose to go with Physics as optional, or many year back one of the aspirant even belonging to engineering background chose to go with philosophy as optional, so it basically depends on various factors such as your interest, availability of study material is too an important factor regarding this.

There are mainly two optional papers, each of 250 marks,  you can choose from the following

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

Other than this, you can also consider literature subjects for optionals such as Assamese,Bengali,Bodo,Dogri,Gujarat,Hindi,Kannada,Kashmiri,Konkani,Maithili,Malayalam,Manipuri,Marathi,Nepali,Oriya,Punjabi,Sanskrit,Santhali,Sindhi,Tamil,Telugu,Urdu,English

UPSC Prelims is generally conducted in June, and Mains is conducted around september-october, last year of course there was a shift in the schedule due to pandemic, if you in graduation then you can anyway start off your preparation in gradual manner such as become acquainted with the syllabus, take a print out of it and go through it on daily basis, this will help you know what to read and skip in terms of books reading or even newspaper reading which is extremely important, so make it a habit to go through newspaper everyday, you can choose among The Hindu or  The Indian Express, start off with NCERTs and then move towards other standard books, go through previous year paper, practice answer writing in advance, apply mock papers and test series on a regular basis, one year is enough to prepare for the exam, but you need to prepare your timeline accordingly especially if you're still in college and you need to balance studies and exam