Which of the following is defined as a substance that tends to produce cancer in an organism?

The term that specifically refers to a substance capable of inducing cancer in living organisms is "carcinogen." Carcinogens can cause changes in cellular structures or DNA that lead to uncontrolled cell multiplication and ultimately cancer development. These substances may be found in various forms, such as chemicals, radiation, or even certain biological agents.

In contrast, other terms in the list pertain to different aspects of biological harm. A mutagen refers to a substance that can cause changes or mutations in the genetic material of an organism, which does not always lead to cancer. A teratogen is a substance that can cause malformations in developing embryos or fetuses, leading to developmental issues rather than cancer. An allergen is a substance that can induce allergic reactions, impacting the immune system rather than causing cancer. Thus, "carcinogen" is the term that directly relates to cancer induction, making it the correct answer in this context.