Explanation:

Think tanks play a crucial role in shaping public policy through research, analysis, and advocacy. Global rankings assess their influence, quality of research, and outreach. \nOption (A) Institute of Defence Studies and Analyses (IDSA): The IDSA, now known as the Manohar Parrikar Institute for Defence Studies and Analyses, is India's foremost think tank on defence, security, and international relations. While highly respected, it was not ranked $20^{th}$ among all think tanks worldwide in the specific context of this question's timeframe. Therefore, this option is incorrect. \nOption (B) Observer Research Foundation (ORF): The Observer Research Foundation (ORF) is an independent global public policy think tank based in New Delhi. In the $2020$ Global Go To Think Tank Index Report (released in $2021$) by the University of Pennsylvania, ORF was indeed ranked $20^{th}$ among the 'Top Think Tanks Worldwide (Non-US)'. Given the question's context, this is the correct answer. Therefore, this option is correct. \nOption (C) Centre for Civil Society (CCS): The Centre for Civil Society is a public policy think tank based in New Delhi, focusing on liberal reforms. While influential in its niche, it has not achieved the $20^{th}$ global ranking among all think tanks. Therefore, this option is incorrect. \nOption (D) Centre for Public Policy Research (CPPR): The Centre for Public Policy Research is an independent public policy think tank based in Kochi, Kerala. It focuses on economic, social, and political issues. It has not achieved the $20^{th}$ global ranking among all think tanks. Therefore, this option is incorrect. \nTherefore, the Observer Research Foundation (ORF) was ranked $20^{th}$ among the top think tanks worldwide (non-US) in a relevant global index.