The eligibility criteria for appearing in the UPSC Civil Services Preliminary Examination include being a citizen of India, or a subject of Nepal or Bhutan. Candidates must also meet specific age limits, usually between 21 to 32 years, and possess a bachelor's degree from a recognized university. Certain relaxations apply for candidates belonging to reserved categories. Furthermore, candidates must fulfill the number of attempts allowed, which varies based on their category. Meeting these requirements is essential to qualify for the rigorous selection process of the UPSC.