The correct answer is option 3 - Dhaka. Dhaka, which is now the capital city of Bangladesh, was a prominent commercial center during the medieval period. It was known for its thriving textile industry, especially its muslin fabric, which was highly prized in international markets.

However, after the mid-eighteenth century, Dhaka`s commercial importance started to decline. Several factors contributed to this decline. One of the significant factors was the shift in political power. During this period, the British East India Company established control over Bengal, and their focus shifted towards developing Calcutta (option 1) as the main commercial center in the region. Calcutta had better access to the sea and a more favorable location for trade.

Additionally, Dhaka also suffered from natural disasters like floods and famines, which negatively impacted its economy. The decline of the Mughal Empire, which had previously been a major patron of the textile industry in Dhaka, also played a role in the city`s downfall.

In conclusion, while Dhaka was once a thriving commercial center, it experienced a decline in its importance after the mid-eighteenth century due to political changes, natural disasters, and the rise of Calcutta as a rival commercial hub.