Which of the following is a key characteristic of a toxic substance?

A key characteristic of a toxic substance is that it can cause health effects. Toxic substances have the potential to harm living organisms when they are absorbed into the body, ingested, inhaled, or come into contact with the skin. These health effects can range from mild symptoms to severe illnesses, depending on the level of exposure and the nature of the substance.

While flammability, corrosiveness, and safe handling practices can be pertinent factors in evaluating the risks associated with certain materials, they do not solely define toxicity. A toxic substance might not be flammable or corrosive but still pose serious risks to health. Therefore, the ability to cause health effects is a definitive trait of a toxic substance, making this choice the most accurate in the context of toxicity.