What was the reason for the extraordinary success of the Congress in…

The correct answer is Option (3) → The all-inclusive nature of the Congress.

Option (1) It was a multi-party contest amidst communal riots — Incorrect. The election was indeed contested by many parties, but that is a reason other parties could have won, not a reason for the Congress’s success; communal riots had nothing to do with it.

Option (2) Support for the Congress came mainly from the urban areas in the Hindi speaking states. — Incorrect. This describes the Bharatiya Jana Sangh, whose support in the early years came mainly from the urban areas in Hindi-speaking states like Rajasthan, Madhya Pradesh, Delhi and Uttar Pradesh. The Congress, by contrast, had spread across the whole country with a network down to the local level.

Option (3) The all-inclusive nature of the Congress. — Correct. The roots of the Congress’s extraordinary success went back to the freedom struggle: as the Congress was till recently a national movement, its nature was all-inclusive, and it also had the ‘first off the blocks’ advantage.

Option (4) The Congress led the agitation to replace English with Hindi as the official language of India. — Incorrect. It was the Bharatiya Jana Sangh that was in the forefront of the agitation to replace English with Hindi as the official language, not the Congress.