Which of the following categories involves substances that are harmful due to their toxic properties?

The category that involves substances harmful due to their toxic properties is poisons. Poisons include various chemicals and materials that can cause harm or death upon ingestion, inhalation, or skin contact. Their harmful effects are often a result of their ability to interfere with biological processes, leading to toxic reactions within living organisms.

In contrast, corrosives are substances that can damage or destroy living tissue and materials through chemical reactions but aren't specifically defined by toxicity in terms of producing harmful systemic effects. Explosives are substances that can cause rapid combustion and forceful reactions, posing physical dangers through blasts rather than toxic effects. Flammable solids, while capable of igniting and causing fires, do not primarily pose a risk due to toxicity. Thus, poisons are specifically identified as harmful because of their inherent toxic properties.