The correct answer is C) Toxic and infectious substances. The International Maritime Dangerous Goods Code assigns dangerous goods into different classes based on their hazardous properties. Class 6 substances are specifically designated as "Toxic and infectious substances" under this code. This includes materials that are poisonous and can cause injury or death if inhaled, ingested, or absorbed through the skin, as well as substances containing infectious agents. The other answer choices do not accurately describe the Class 6 designation. Class 1 covers explosives, Class 2 covers gases, Class 3 covers flammable liquids, and Class 5 covers oxidizing substances and organic peroxides. Class 6 is specifically reserved for toxic and infectious materials.