Which of the following sexually transmitted disease is not caused by a virus ?

The correct option is Gonorrhoea.

To understand why this is the correct answer, let us look at the causative agents (pathogens) responsible for each of the sexually transmitted diseases (STDs) mentioned in the options:
1. Gonorrhoea: This disease is caused by the bacterium Neisseria gonorrhoeae. Because it is of bacterial origin, it is treated and managed using antibiotics rather than antiviral medications.
2. Genital herpes: This infection is caused by the Herpes Simplex Virus (typically HSV-2, and sometimes HSV-1).
3. Genital warts: These are caused by certain strains of the Human Papillomavirus (HPV).
4. Hepatitis-B: This is a serious liver infection caused by the Hepatitis B Virus (HBV).

Since Genital herpes, Genital warts, and Hepatitis-B are all caused by viruses, Gonorrhoea is the only disease in the list that is not caused by a virus (it is bacterial).