The correct answer is (a) N.K. Singh.
N.K. Singh is an Indian economist and politician who served as the 15th Finance Commission of India from 2017 to 2020. He is a member of the Rajya Sabha, the upper house of the Indian Parliament, and is a former member of the Planning Commission of India.
Hasmukh Adhia is an Indian civil servant who served as the Finance Secretary of India from 2015 to 2017. He is a former member of the Reserve Bank of India and the Securities and Exchange Board of India.
R.S. Sharma is an Indian civil servant who served as the Chairman of the Telecom Regulatory Authority of India from 2015 to 2017. He is a former member of the Indian Administrative Service and has held various positions in the government of India.
Rajeev Kumar is an Indian police officer who served as the Director of the Central Bureau of Investigation from 2017 to 2019. He is a former member of the Indian Police Service and has held various positions in the police force of India.