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

Modern Indian History Developments

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

Peasant resistance to the oppressive conduct of planters

Its unprofitability in the world market because of new inventions

National leaders’ opposition to the cultivation of indigo

Government control over the planters

Colonial rulers exploited the knowledge of Indigo plantation in India and use it to benefit themselves especially in Bengal.
Indigo culture became modernised under the British. The agricultural indigo as a product of economy sprouted, grew and later completely disappeared with the advent of synthetic dyes in a matter of just a century and half.
The synthetic dye was much cheaper and blew the final blow to the natural plant produced indigo dye and indigo crop became a part of history. The successful commercialisation of the synthetic type in the late nineteenth and early twentieth centuries was able to interrupt and redirect the course of colonial history of indigo production.
Now most of the indigo dye used world-over is made synthetically.

Hence option 2nd is correct.