The correct answer is option 'D' Janata Party.

Explanation:
The Janata Party was a political party formed in 1977 as a result of the merger of several opposition parties. It was a coalition of various political groups that came together to challenge the dominance of the ruling Congress Party.

B.P. Mandal, also known as Bindeshwari Prasad Mandal, was an Indian parliamentarian and social reformer. He played a significant role in the formation of the Janata Party. Mandal was a key leader of the backward classes and fought for their rights and representation in politics.

Mandal's involvement with the Janata Party was primarily motivated by the party's commitment to social justice and empowerment of marginalized communities. The Janata Party emerged as a strong alternative to the Congress Party, which had been in power for several years.

The Janata Party's formation marked a turning point in Indian politics as it provided a platform for various opposition parties to unite and challenge the Congress Party's hegemony. It was a broad-based coalition that included ideologically diverse groups ranging from socialists to right-wing organizations.

The Janata Party's primary objective was to restore democracy and address issues of corruption, nepotism, and authoritarianism that had plagued the Congress Party's rule. The party's formation coincided with the lifting of the Emergency, which was imposed by then Prime Minister Indira Gandhi.

In the 1977 general elections, the Janata Party achieved a significant victory, ending the Congress Party's uninterrupted rule. The party formed the government with Morarji Desai as the Prime Minister. This marked the first non-Congress government at the center since India's independence in 1947.

Overall, B.P. Mandal's decision to join the Janata Party in 1977 was a strategic move to bring together various opposition forces and challenge the Congress Party's dominance. His involvement was instrumental in mobilizing support from the backward classes and ensuring their representation in the political landscape.