The main challenge faced by candidates in the essay paper of UPSC Mains is the ability to effectively articulate thoughts within a limited timeframe. Candidates must select relevant topics and develop a coherent, structured argument that showcases their analytical skills and depth of understanding. Managing time while ensuring clarity and cohesiveness can be daunting. Additionally, candidates are often required to draw on a wide range of knowledge and experiences, which necessitates thorough preparation and practice in diverse subjects to enhance their essay-writing capabilities.