Answer by taking a quote from the following article:

Sarvepalli Radhakrishnan listen (5 September 1888 - 17 April 1975) was an Indian philosopher and statesman who was the first Vice President of India (1952-1962) and the second President of India from 1962 to 1967. One of India's most distinguished twentieth-century scholars of comparative religion and philosophy, his academic appointments included professor of Philosophy at the University of Mysore (1918-1921), the King George V Chair of Mental and Moral Science at the University of Calcutta (1921-1932) and Spalding Professor of Eastern Religion and Ethics at University of Oxford (1936-1952). His philosophy was grounded in Advaita Vedanta, reinterpreting this tradition for a contemporary understanding.

Radhakrishnan was awarded scholarships throughout his academic life. He joined Voorhees College in Vellore but switched to the Madras Christian College at the age of 17. He graduated from there in 1906 with a master's degree in Philosophy, being one of its most distinguished alumni. Radhakrishnan studied philosophy by chance rather than choice. Being a financially constrained student, when a cousin who graduated from the same college passed on his philosophy textbooks in to Radhakrishnan, it automatically decided his academic course. Radhakrishnan wrote his thesis for the M.A. degree on "The Ethics of the Vedanta and its Metaphysical Presuppositions". It "was intended to be a reply to the charge that the Vedanta system had no room for ethics." He was afraid that this M.A. thesis would offend his philosophy professor, Dr. Alfred George Hogg. Instead, Hogg commended Radhakrishnan on having done most excellent work. Radhakrishnan's thesis was published when he was only twenty. According to Radhakrishnan himself, the criticism of Hogg and other Christian teachers of Indian culture "disturbed my faith and shook the traditional props on which I leaned." Radhakrishnan himself describes how, as a student, The challenge of Christian critics impelled me to make a study of Hinduism and find out what is living and what is dead in it. My pride as a Hindu, roused by the enterprise and eloquence of Swami Vivekananda, was deeply hurt by the treatment accorded to Hinduism in missionary institutions. This led him to his critical study of Indian philosophy and religion and a lifelong defence of Hinduism against "uninformed Western criticism".

What did he study?
Radhakrishnan studied philosophy by chance rather than choice.