Gandhi`s first venture in all-India politics was indeed the Champaran Movement (Option 3). This significant event occurred in 1917 when Gandhi traveled to Champaran in Bihar to assist the local peasants who were being forcibly made to grow indigo by British planters. This was Gandhi`s first active involvement in Indian politics on a national scale, using his principles of Satyagraha or passive resistance.

Option 1, Non-Cooperation Movement, took place later in 1920. It was a significant political movement led by Gandhi, but it was not his first venture in all-India politics.

Option 2, the Rowlatt Satyagraha, was a protest against the Rowlatt Act enacted by the British government in 1919. This too was a crucial moment in Gandhi`s political career, but not his first.

Option 4, the Dandi March, refers to the Salt Satyagraha of 1930 when Gandhiji undertook a 240-mile march to the Arabian Sea to protest against the British salt tax. Though it was a major event, it was not his first all-India political venture.