Who has been the only Lok Sabha Speaker to have become the President of India ?

Neelam Sanjiva Reddy is the only individual in Indian history to have served as the Speaker of the Lok Sabha and subsequently become the President of India [1]. He served two non-consecutive terms as Speaker: first from 1967 to 1969 during the Fourth Lok Sabha, and again in 1977 during the Sixth Lok Sabha [1]. He resigned from the Speakership in 1969 to contest the Presidential election (which he lost) and resigned again in 1977 after being nominated for the Presidency. In 1977, he achieved the distinction of being elected unopposed as the sixth President of India [2]. While other options like Giani Zail Singh, V.V. Giri, and K.R. Narayanan held various high offices such as Chief Minister or Vice-President, none of them served as the Speaker of the Lok Sabha.

Sources

1. [1] https://www.thehindu.com/news/national/sanjiva-reddy-created-many-records/article3551790.ece
2. [2] Indian Polity, M. Laxmikanth(7th ed.) > Chapter 23: Parliament > Table 23.6 Durations of the Lok Sabha (from First Lok Sabha to Present i lok Sabha) > p. 266