Concept:A sexually transmitted disease is caused by pathogens spread through sexual contact.Explanation:Treponema pallidum is the bacterium that causes syphilis, which is a sexually transmitted disease.Entamoeba histolytica causes amoebic dysentery.Salmonella typhi causes typhoid fever.Clostridium tetani causes tetanus.Therefore, the agent of a sexually transmitted disease is Treponema pallidum.Answer:C. Treponema pallidum