hai kunal,
this is setty.you have said that you are a computer science student.in civil services you better take the optional yourself.optionals are taken by ones own interest.they should not be taken by seeing others optionals.first of all for your intimation there are 23 optionals in prelims and 25 optionals in mains.i will list out them for you.

List of Optional Subjects CS (Preliminary) - Total 23

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

List of Optional Subjects - CS (Main) - (Total 25)

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

now you have the clear idea about the subjects involved in civils.for your interest you may opt for subjects like public administration,sociology,physcology,geography,indian history ,political science,anthropology etc for your first optional.they are little advantageous than others.but dont take them for granted.and for your second optional you can take the first optional the one you took in prelims and another subject like maths ,physics ,chemistry something like that because you already have studied them.but these will take time.however you have ample time for your first attempt.here one thing i must emphasis that every subject has the same weigtage in civils .there would be no partiality in giving more advantage to one subject than to another .ultimately it should be your own decision .