Indigo cultivation in India declined by the beginning of the 20th century because of

The correct answer is B. Indigo lost its market because synthetic indigo, first synthesised by Adolf von Baeyer in the 1880s and produced commercially by BASF from around 1897, could be made far more cheaply than natural indigo. Once it flooded the world market, Indian indigo became unprofitable for both planters and cultivators.

Why the others fail:

• A Peasant resistance was real, as in the 1859-60 indigo revolt in Bengal, but it was a response to the tinkathia and planter oppression. It did not make indigo unprofitable, and the decline came from the market.
• C Nationalist opposition came mainly after the decline had begun. Gandhi's Champaran satyagraha (1917) took place when synthetic dye had already undermined the trade.
• D The colonial government generally backed the planters. The Indigo Commission (1860) and later measures sought to manage disputes, not to suppress cultivation.

Core concept: the Indian indigo decline is a case of colonial commercial agriculture collapsing under external market shocks, a useful theme for Mains on the economic drain and the peasant economy.

Trap: Champaran (1917) is the famous episode, so aspirants pick A or C. But the market collapse had already come first, and Champaran was a protest against planter exploitation, not the cause of the decline.