You cannot file RTI for Copies of Answer Sheet of Selected Candidates of Civil Mains from Union Public Service Commission (UPSC).
Matters pertaining to Exam Papers are termed as Third Party information and are exempted under Section 8 (1) (j) of the RTI Act 2005. You can file RTI to obtain information pertaining to the Marks obtained by the Selected Candidates whose Result has been Published by UPSC. You can also file RTI to obtain Copies of your Answer Sheet.
UPSC is a premier Central Recruiting Agency responsible for conducting examinations for All India Services and group A & group B of Central services. Its was established comes under Department of Personnel and Training. UPSC is a "Public Authority" as per Section 2 (h) of the Act.
To know more about the procedure to file RTI Applications, please refer https://onlinerti.com/