The remarkable success of the Congress party can be traced back to its deep roots in the legacy of the freedom struggle. Being viewed as the rightful heir of the national movement, the Congress party attracted leaders who had actively participated in the battle, many of whom contested elections as Congress candidates. With its well-established organization, the Congress party had already initiated its campaign while other parties were still strategizing. In fact, several parties were formed either around Independence or after that, giving the Congress party a significant advantage as it was the first to embark on political endeavours.

Assertion:  Congress was not only an ideological coalition but a social coalition as well.
Reason:  Peasants and industrialists, urban dwellers and villagers, workers and owners, middle, lower and upper classes, and castes, all found space in the Congress.

Both the Assertion and the Reason are correct and the Reason is the correct explanation of the Assertion.

Congress as a social coalition & as an ideological coalition:

Social Coalition:
The Congress party evolved from a pressure group for the educated, professional, and commercial classes in 1885 to a mass movement in the twentieth century.
Its initial composition was dominated by English-speaking, upper-caste, upper-middle-class, and urban elites.
However, with each civil disobedience movement, the Congress party expanded its social base and brought together diverse groups with often contradictory interests.
It included peasants, industrialists, urban and rural dwellers, workers, owners, and people from various social classes and castes.
Over time, the Congress party's leadership expanded beyond the upper caste and upper-class professionals to include leaders with a rural orientation and agricultural backgrounds.
By the time of independence, the Congress party had transformed into a social coalition representing India's diversity in terms of classes, castes, religions, languages, and various interests.
While some groups merged their identity within the Congress, others maintained their distinct beliefs and continued to exist within the party.

Ideological coalition:
The Congress party served as an ideological coalition, accommodating individuals and groups with a range of beliefs, including revolutionaries, pacifists, conservatives, radicals, extremists, moderates, and those from the left, right, and centre.
It functioned as a platform for numerous groups, interests, and even political parties to participate in the national movement.
During the pre-independence era, many organizations and parties with their own constitutions and organizational structures were allowed to operate within the Congress.
Some of these groups, such as the Congress Socialist Party, later separated from the Congress and became opposition parties.
Despite differences in methods, specific programs, and policies, the Congress party managed to contain and, to some extent, resolve conflicts and build consensus.