The option to choose an additional subject is provided during the UPSC mains examination in the optional subjects paper. Candidates can select one optional subject from a list of nearly 48 subjects. This allows aspirants to tailor their preparation based on their strengths and interests, thereby enhancing their performance. The choice of an optional subject can significantly impact overall scores, making it a critical decision for candidates aiming to maximize their chances of success in the UPSC civil services examination.