Explanation:

- Correct Answer: Option 2, April

- Gandhi arrived in Champaran in April 1917.

- This visit marked the beginning of his active participation in India’s struggle for independence.

- Champaran was the site of Gandhi’s first nonviolent protest for India's farmers.

- Option 1, March

- Gandhi did not arrive in Champaran in March 1917.

- No significant events related to his arrival are noted for this month.

- Option 3, July

- By July 1917, Gandhi was already involved in activities in Champaran.

- It wasn't the month of his initial arrival.

- Option 4, August

- Also too late for his arrival in 1917.

- By this time, Gandhi's movement in Champaran had already gathered momentum.