Among the options provided, the individual who served as the Chief Economist of the International Monetary Fund (IMF) is Raghuram Rajan. Here is a detailed reasoning explaining why:

1. Ashok Lahiri: Ashok Lahiri is an Indian economist, but he did not serve as the Chief Economist of the IMF. Lahiri has held various important positions in India, including Chief Economic Adviser to the Government of India and Chairman of the Prime Minister`s Economic Advisory Council. However, he has no direct association with the IMF as its Chief Economist.

2. Sumantra Ghoshal: Sumantra Ghoshal was an influential management thinker and a professor of Strategic and International Management at the London Business School. While Ghoshal made significant contributions to the field of management, he did not serve as the Chief Economist of the IMF. His expertise focused on organizational behavior and strategy, rather than macroeconomics or international finance.

3. Saumitra Chaudhuri: Saumitra Chaudhuri was an Indian economist and served as the Member of the Planning Commission of India from 2009 to 2014. However, he also did not serve as the Chief Economist of the IMF. Chaudhuri`s expertise was primarily in the areas of economic policy, planning, and development, rather than being directly associated with the IMF.

4. Raghuram Rajan: Raghuram Rajan, an Indian economist, did serve as the Chief Economist of the IMF. He held this position from 2003 to 2006. During his tenure, Rajan made notable contributions to the IMF`s research and policy work. He is well-known for his early warnings about the global financial crisis and his research on banking and financial sector reforms. After leaving the IMF, Rajan went on to serve as the Governor of the Reserve Bank of India from 2013 to 2016. His extensive experience and expertise in both academia and policymaking have made him a prominent figure in the field of economics.

In summary, Raghuram Rajan served as the Chief Economist of the International Monetary Fund (IMF), making him the correct option among the choices provided. His contributions to the IMF and his subsequent roles in academia and policymaking further validate his expertise in the field of economics.