Which substance is known for being both flammable and toxic that is commonly transported on tankers?

Benzene is a highly flammable liquid that is also considered toxic to humans. It is a colorless chemical that is widely used in the chemical industry, particularly as a precursor to the production of various chemicals and plastics. Benzene poses health hazards through inhalation, ingestion, or skin contact, and prolonged exposure can lead to serious health issues, including cancer.

In the context of transportation on tankers, the properties of benzene necessitate strict safety protocols to prevent leaks and spills, as well as to protect the crew and the environment. This is key when assessing the substances transported on tankers, as safety measures are critical for handling materials with both flammable and toxic characteristics.

The other options provided do not share these same properties. Water is non-flammable and non-toxic. Salt is generally safe and used in various applications but also does not exhibit flammability or toxicity. Sugar, while it can combust under certain conditions, is not considered toxic and is commonly regarded as safe for consumption. Therefore, benzene is the substance that best fits the description of being both flammable and toxic within the context of materials frequently transported on tankers.