The correct answer is 'a) Surat'. In the 18th century, Surat witnessed a decline as a major port town in India. Let's explore the reasons behind this decline in detail:

1. Emergence of Bombay:
- During the 17th century, the British East India Company established a factory in Surat, which grew into a major trading post.
- However, in the late 17th century, the British East India Company shifted its headquarters from Surat to Bombay (now Mumbai) due to the favorable geographical location and better infrastructure of Bombay.
- This shift led to a decline in Surat's prominence as a trading center.

2. Rise of Bombay:
- Bombay's natural harbor provided a safe and convenient location for ships, allowing for easier trade and navigation.
- The British East India Company developed Bombay as a major port and established various facilities such as warehouses, docks, and shipbuilding yards.
- The company also implemented favorable trade policies and provided incentives for merchants to settle in Bombay, attracting traders from Surat and other regions.
- These developments led to the rise of Bombay as a major port town, overshadowing Surat's significance.

3. Decline in Surat's trade:
- The shift in the British East India Company's headquarters and the rise of Bombay resulted in a decline in Surat's trade.
- Many merchants and traders moved to Bombay, seeking better opportunities and facilities.
- Surat also faced competition from other emerging port towns like Calcutta (now Kolkata) and Madras (now Chennai), which further contributed to its decline.

4. Loss of monopoly:
- Surat was known for its textile industry, particularly silk and cotton textiles, which were in high demand in Europe and other parts of Asia.
- However, with the decline of Surat, other regions like Bengal and Tamil Nadu began to dominate the textile trade, leading to a loss of Surat's monopoly in this industry.

In conclusion, Surat witnessed a decline in the 18th century due to the emergence of Bombay as a major port town, better infrastructure and trade facilities in Bombay, competition from other port towns, and the loss of its monopoly in the textile industry.