• Gonorrhea is caused by Neisseria gonorrhoeae — correct.
• Genital herpes is caused by Herpes simplex virus (HSV), not Human papilloma virus (HPV) — this is the mismatched pair.
• AIDS is caused by HIV — correct.
• Syphilis is caused by Treponema pallidum — correct.
• Hence the mismatched pair is (2).