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

(a) peasant resistance to the oppressive conduct of planters
(b) its unprofitability in the world market because of new inventions ✓ UPSC's answer
(c) national leaders' opposition to the cultivation of indigo
(d) Government control over the planters

Why the answer is (b)

• Indigo cultivation collapsed after 1897 when German chemists (BASF, following Adolf von Baeyer's work) commercialised synthetic indigo, which was cheaper and more uniform — option (b).

• Indian indigo exports fell from about 19,000 tonnes in the 1890s to a fraction by 1914; Champaran planters were already in decline when Gandhi arrived in 1917.

• Peasant resistance (a) — the 1859–60 Indigo Revolt — hurt Bengal planters but production simply shifted to Bihar; national leaders (c) and government control (d) were not the cause of the industry's decline.

• Hence option (b).

Why the other options are wrong

(a) peasant resistance to the oppressive conduct of planters: Peasant revolts shifted cultivation to Bihar but did not end it.
(c) national leaders' opposition to the cultivation of indigo: National leaders opposed planter oppression, not cultivation itself.
(d) Government control over the planters: There was no effective government control over planters.

Asked in the GS Paper I of the UPSC Civil Services Preliminary Examination 2020, held on 4 October 2020. Question and answer key: Union Public Service Commission. Explanation: UPSC Answer Check.