How is a portable tank different from a cargo tank?

A portable tank is defined as a tank designed to transport liquids and is not permanently attached to a vehicle, allowing for it to be easily removed and transferred. This characteristic enables flexibility in transporting various materials since the tank can be moved independently of the vehicle.

On the other hand, a cargo tank is specifically built to be permanently mounted to a vehicle, serving as part of its structure for transporting bulk liquids and gases. This distinction is important for both safety regulations and the management of different types of cargo.

The option that states portable tanks are not permanently attached to the vehicle accurately captures this distinction, highlighting the versatility of portable tanks in transportation compared to the fixed nature of cargo tanks.