The popularity of optional subjects in CSS exams may vary from year to year based on factors such as current trends, candidates' interests, and scoring potential. However, some subjects have consistently been popular among CSS aspirants. Here are a few examples:

1. International Relations: This subject deals with the study of international politics, diplomacy, global issues, and international organizations.

2. Political Science: It focuses on the study of political systems, theories, governance, public administration, and political ideologies.

3. Public Administration: This subject covers topics related to administrative systems, public policy, bureaucracy, and governance.

4. History: It involves the study of world history, regional history, and the historical events and developments that have shaped societies.

5. Sociology: This subject explores the study of society, social structures, social institutions, and social behaviors.

6. English Literature: It involves the study of literary works, including novels, poetry, plays, and literary analysis.

7. Psychology: This subject covers various aspects of human behavior, mental processes, personality, and psychological theories.

8. Economics: It focuses on economic principles, theories, macroeconomics, microeconomics, and economic policies.

9. Law: This subject covers topics related to constitutional law, international law, criminal law, and legal principles.