Which of the following is defined as a cancer-causing substance?

The correct choice is "carcinogen," which specifically refers to any substance or agent that is capable of causing cancer in living tissue. Carcinogens can be chemical substances, viruses, or other factors that lead to the development of cancer by causing mutations in the DNA or promoting the growth of cancerous cells. Identifying carcinogens is crucial in fields such as public health, occupational safety, and environmental regulations, especially for workers who may be exposed to these harmful agents.

The other terms, while related to health risks, do not specifically pertain to cancer-causing properties. Toxins refer to harmful substances produced by living organisms, which may not necessarily cause cancer. Mutagens are agents that cause genetic mutations but do not always lead to cancer. Contaminants broadly refer to unwanted substances present in the environment or materials, but they do not specifically indicate a cancer risk. Understanding these distinctions is key to recognizing how different agents can impact health, particularly in occupations exposed to hazardous materials such as asbestos.