Which of the following inorganics can lead to illness when found in water?

Lead is a highly toxic heavy metal that can lead to significant health issues when present in water, particularly through sources such as the corrosion of plumbing systems and lead pipes. When ingested, lead can accumulate in the body and negatively impact various systems, especially in children, leading to developmental problems, behavioral issues, and cognitive impairment.

In contrast, the other listed inorganics—calcium, sodium, and magnesium—are generally considered essential minerals for human health. While excessive amounts of sodium can contribute to hypertension and other health concerns, it doesn’t have the same level of toxicity associated with lead. Calcium and magnesium are important for bone health and various bodily functions and are not typically regarded as contaminants in water to the same harmful extent as lead. Thus, lead's potential to cause illness when found in water makes it the correct response in this context.