Question

What did incident that prompted gandhi to raise his voice of protest against the british?

Answer 1

It was due to the dogged determination of an illiterate but resolute sharecropper from Champaran called Rajkumar Shukla which helped Gandhi make the decision of urging the British to leave India. 

Answer 2

The Champaran Satyagraha is the right answer.

Initiated by M. K. Gandhi the Champaran movement was a major peasant's uprising that took place in the Champaran district of Bihar in the year 1917. This movement began when a peasant name Rajkumar Shukla came to Gandhi with the problem of exploitation in his area, and Gandhi decided to launch a movement against the exploitation of Indian farmers in Champaran. The main reason for this uprising was that the peasants were hardly receiving any payment in return to grow opium. Thus Gandhi decided to launch a Satyagraha to defend the peasants from exploitation.