What is the only method that is considered 100% effective in preventing STIs?

The method that is considered 100% effective in preventing sexually transmitted infections (STIs) is abstaining from all sexual activities. This complete avoidance eliminates any potential risk of transmission that could arise from sexual contact.

All other methods, such as consistent condom use or dental dams, can significantly reduce the risk of STIs but cannot completely eliminate it. Factors such as improper usage, breakage, or slippage can compromise their effectiveness. Additionally, receiving regular medical check-ups can help in early detection and treatment of STIs but does not prevent them from occurring in the first place.

Therefore, choosing not to engage in any sexual activities is the only way to ensure absolute protection against STIs, making it the correct answer in this context.