What is the primary feature of a tank car?

The primary feature of a tank car is its design, which is specifically engineered to haul bulk liquids. This type of railcar is constructed with a cylindrical tank that can safely contain and transport various liquid commodities, including chemicals, petroleum, and food-grade products. The tank is typically made from strong materials to withstand the pressure and corrosive nature of some liquids. In contrast, other choices refer to functionalities that do not align with the primary purpose of a tank car; for example, solid materials require different types of freight cars such as boxcars or hoppers, while transporting people is handled by passenger cars. Distributing freight is a broader term that applies to various railcar types, but tanks are specialized for liquid transport specifically. Thus, option C accurately reflects the core characteristic of tank cars.