Port cities are cities that are located on a coast or along a river and have a harbor or port that serves as a point of entry and departure for ships and boats. They are important hubs for trade and commerce, facilitating the movement of goods and people between different regions and countries.

The correct answer to the given question is option 'B', Chennai. Chennai, formerly known as Madras, is a major port city located on the eastern coast of India, in the state of Tamil Nadu. It is the capital city of Tamil Nadu and one of the largest urban centers in India.

Below, I will provide a detailed explanation of why Chennai is considered a port city:

1. Geographic Location:
- Chennai is situated along the Coromandel Coast of the Bay of Bengal.
- Its strategic location makes it an ideal port city, providing access to both domestic and international trade routes.

2. Chennai Port:
- Chennai has a well-developed port, the Chennai Port, which is one of the oldest and busiest ports in India.
- The port handles a wide range of cargo, including containers, automobiles, petroleum products, and bulk commodities.
- It plays a crucial role in facilitating trade and commerce for the region and acts as a gateway for imports and exports.

3. Economic Significance:
- The presence of a port has contributed to Chennai's economic growth and development.
- It has attracted various industries, including manufacturing, automobile, technology, and services, which benefit from the port's connectivity and logistical advantages.
- The port also generates employment opportunities and revenue for the city and the state.

4. Trade and Connectivity:
- Chennai Port serves as a vital link in international trade, connecting India with various countries across the globe.
- It handles both imports and exports, facilitating the exchange of goods and fostering international relations.
- The port's connectivity to road, rail, and air networks further enhances its significance and accessibility.

In conclusion, Chennai is a port city because of its geographical location along the coast, the presence of a well-established port, its economic significance, and its role in facilitating trade and connectivity.