Question

what is the only guaranteed method of preventing pregnancy or contracting an sti?

Explanation:

Abstinence means refraining from sexual activity. Pregnancy occurs through sexual intercourse (in most cases) and STIs (sexually transmitted infections) are mainly spread via sexual contact. By avoiding sexual activity (abstinence), there is no chance of sperm fertilizing an egg (preventing pregnancy) and no exchange of bodily fluids that could transmit STIs.

Answer:

Abstinence.