Who among the following had the longest tenure as the Prime Minister of India ?

Among the options provided, Morarji Desai had the longest tenure as the Prime Minister of India. He served from March 24, 1977, to July 28, 1979, which is approximately 2 years and 126 days [c1][t1]. In comparison, Lal Bahadur Shastri served from June 9, 1964, to January 11, 1966, a period of about 1 year and 216 days [c1]. Vishwanath Pratap Singh (V.P. Singh) held office from December 2, 1989, to November 10, 1990, lasting less than a year (343 days) [t1][t7]. Chaudhary Charan Singh had the shortest tenure among the four, serving for only 170 days from July 28, 1979, to January 14, 1980 [t1]. While Jawaharlal Nehru holds the record for the overall longest tenure in Indian history, within this specific list, Morarji Desai's term was the most extensive [t2][t5].

Sources

1. [1] Introduction to the Constitution of India, D. D. Basu (26th ed.). > Chapter 35: TABLES > C. PRIME MINISTERS OF INDIA > p. 534
2. [2] https://www.pmsangrahalaya.gov.in/prime-ministers-of-india
3. [3] https://knowindia.india.gov.in/general-information/prime-ministers-of-india.php