Indigo, also known as "Blue Gold", was a highly profitable commodity in the European market during the 19th century. However, by the beginning of the 20th century, its cultivation in India declined significantly.

The main reason for the decline in indigo cultivation was its unprofitability in the world market due to new inventions. Synthetic dyes were invented in the late 19th century, which were cheaper and easier to produce than natural indigo. This led to a drop in the demand for natural indigo, making its cultivation unprofitable.

Therefore, the correct answer is B.