Final Answer:
The abbreviation for any disease transmitted through sexual activity is:
a. STI
Step-by-step explanation:
STI stands for sexually transmitted infection, which is a term used to describe any infection that is spread through sexual contact. This includes infections caused by bacteria, viruses, parasites, and other microorganisms. Some common STIs include chlamydia, gonorrhea, syphilis, herpes, and human papillomavirus (HPV). It is important to practice safe sex and get tested regularly if you are sexually active to prevent the spread of STIs and to receive prompt treatment if you do become infected.