Explanation

Option B is correct: In the late 19th century, German chemical scientists, led by Adolf von Baeyer and industrial synthesis teams at BASF and Hoechst, successfully developed and commercially produced synthetic chemical indigo (derived from coal-tar/aniline).

– The synthetic dye was dramatically cheaper, more consistent in shade, and easier to manufacture in massive industrial volumes compared to natural plant-extracted indigo.
– Consequently, international market demand for Indian natural indigo collapsed, making natural indigo plantations economically unviable and unprofitable by the dawn of the 20th century.
– Peasant resistance (such as the Indigo Revolt of 1859-60) and later the Champaran Satyagraha of 1917 highlighted the exploitative tinkathia system, but the definitive global economic cause of industry-wide decline was scientific obsolescence.

Therefore, the decline was caused by its unprofitability in the world market because of new inventions — Option B.