Trichomoniasis is a STD whereas other are not.
HIV is a pathogen whereas others are diseases.
Gonorrhoea is a viral disease whereas others are bacterial.
Hepatitis B is eradicated completely whereas others are not.
HIV is a retrovirus that causes the disease AIDS. Hepatitis B is a viral disease, gonorrhea is a bacterial disease and trichomoniasis is a protozoan disease. Thus, the correct answer is option C.