The correct answer is option 1, Morarji Desai.

Morarji Desai had the longest tenure as the Prime Minister of India. He served as the Prime Minister from 1977 to 1979. During his term, he implemented several economic and social reforms, such as nationalizing banks, promoting agricultural development, and advocating for decentralization of power. Desai was known for his austerity measures and his efforts to improve relations with neighboring countries. He was a member of the Janata Party and became the first non-Congress Prime Minister of India.

Option 2, Charan Singh, was the Prime Minister of India for a brief period in 1979. He served as the Prime Minister for only a month, making him the shortest-serving Prime Minister in the history of India.

Option 3, V.P. Singh, served as the Prime Minister of India from 1989 to 1990. He implemented the Mandal Commission recommendations, which aimed to provide reservations for socially and educationally backward classes. His term was marked by political instability and he resigned from office after losing his majority in Parliament.

Option 4, Lal Bahadur Shastri, was the Prime Minister of India from 1964 to 1966