The decline of indigo cultivation in India at the start of the 20th century is attributed to its unprofitability in the world market due to new inventions (Option 2).

Discussing the options,

Option 1: Peasant resistance to the oppressive conduct of planters could have added pressure but it is not the primary reason for the decline. The planters could still have continued cultivating indigo if it remained profitable.

Option 3: While national leaders might have dissuaded agriculture of indigo, again, if it was profitable, the practice would have continued.

Option 4: Government control over the planters might have impacted the profitability but it is not identified as the main cause.

The key reason was the market becoming unfavorable (Option 2). The invention of synthetic dyes reduced global demand for natural indigo, undercutting its price and thus prompting planters to switch to other crops. The economics of demand and supply prevailed and led to the decline.