STIs which is not curable is Genital herpes. Thus, option "C" is correct.
Genital herpes is not curable since its is a viral disease caused by a virus called herpes simplex virus type one (HSV-1) and type two (HSV-2). It can be spread from the mouth to the genitals through oral Intercourse.
However there medications that help in management of genital herpes, treatment of a viral infection is difficult because viruses are tiny and reproduce within cells.
Thus, option "C" is correct.
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