Assertion: Indian merchants and middlemen supplied and participated in the trade of opium and they helped integrate Bombay's economy directly into Malwa, Rajasthan and Sind where opium was grown.
Reason: Bombay's capitalists came from diverse communities such as Parsi, Marwari, Konkani Muslim, Gujarati Bania, Bohra, Jew and Armenian.

Both the Assertion and the Reason are correct but the Reason is not the correct explanation of the Assertion.

Bombay was initially seven islands. As the population grew, the islands were joined to create more space and they gradually fused into one big city. Bombay was the commercial capital of colonial India. As the premier port on the west coast, it was the centre of international trade.
By the end of the nineteenth century, half the imports and exports of India passed through Bombay. One important item of trade was opium that the East India Company exported to China. Indian merchants and middlemen
supplied and participated in this trade and they helped integrate Bombay's economy directly into Malwa, Rajasthan and Sind where opium was grown.
This collaboration with the Company was profitable and led to the growth of an Indian capitalist class.
Bombay's capitalists came from diverse communities such as Parsi, Marwari, Konkani Muslim, Gujarati Bania, Bohra, Jew and Armenian.