How is a tank vessel defined?

A tank vessel is specifically defined as a type of vessel that is constructed to carry hazardous materials in bulk. This definition is crucial because it highlights the unique design and safety features that tank vessels must possess to transport substances that can be dangerous to the environment and public safety.

Tank vessels are built with specialized tanks that are designed to safely hold large quantities of liquids, which can include oil, chemicals, and other hazardous materials. These ships often feature advanced safety systems, ventilation, and containment measures to prevent spills or leaks during transport.

The alternative definitions provided in the other options do not hold true for tank vessels. For instance, a vessel designed for passenger transport is typically referred to as a passenger ship or ferry, rather than a tank vessel. Moreover, while many tank vessels are focused on liquid cargo, the primary characteristic is their ability to handle hazardous materials, not simply any liquid. Finally, vessels for dry bulk cargo are entirely different and categorized separately, as they are not built for carrying liquids at all. Therefore, the reasoning for option B being the correct answer lies in its alignment with the essential characteristics and regulatory definitions surrounding tank vessels in the maritime industry.