Which combining form denotes something that is cancerous?

The combining form that denotes something that is cancerous is "carcin/o." In medical terminology, this prefix is utilized to refer specifically to cancer or cancerous conditions. "Carcinoma" is a type of cancer that begins in the skin or in tissues that line or cover internal organs, emphasizing the association between this combining form and malignancy.

The other combining forms have distinct meanings: "cyt/o" refers to cells, "derm/o" pertains to skin, and "dent/o" relates to teeth. Therefore, while these forms are essential in understanding various medical terms, only "carcin/o" directly indicates a connection to cancer, distinguishing it as the correct answer in the context of the question.