Final answer:
The skull and crossbones symbol in WHMIS classification signifies 'toxicity,' indicating a substance that can cause serious health effects or death. It does not directly relate to corrosive, biohazardous, or infectious materials.
Step-by-step explanation:
The first four categories represented by the skull and crossbones symbol in WHMIS (Workplace Hazardous Materials Information System) specifically refer to the hazardous waste classification mentioned by both the US Environmental Protection Agency (EPA) and WHMIS. These categories do not directly relate to the options provided (a) Toxic materials, (b) Corrosive materials, (c) Biohazardous materials, d) Infectious materials). Instead, characteristic hazardous wastes are identified as materials that are hazardous due to their ignitability, corrosivity, reactivity, or toxicity. Specifically, the skull and crossbones symbol in WHMIS indicates toxicity, warning that a substance can cause serious health effects or death if inhaled, ingested, or absorbed through the skin.