Which of the following sexually transmitted diseases is not completely curable?

Correct option is C. Genital herpes. Genital herpes is caused by type-II-herpes simplex virus. At present there is no way to control infection by type II herpes simplex virus and therefore, genital herpes, the disease caused by the virus, cannot be cured. Other non-curable STIs are hepatitis-B and HIV. So, the correct answer is 'Genital herpes'