Final answer:
The only completely reliable method for preventing conception is abstinence, which is the absence of sexual intercourse. Surgical options like tubal ligation and vasectomy are permanent but other contraceptive methods require adherence to usage guidelines.
Step-by-step explanation:
The only completely reliable method for preventing backflow, which in this context means preventing conception or pregnancy, is abstinence. This means not engaging in sexual intercourse, and therefore, there is no chance for sperm to meet an egg and result in pregnancy. While other contraceptive methods like tubal ligation and vasectomy are considered permanent methods of prevention, they are surgical procedures and carry their own risks and considerations. However, the effectiveness of a contraceptive method is also dependent on proper and consistent use, known as compliance with the contraceptive method. Non-permanent, or reversible, methods provide contraception for a short-term duration but require adherence to usage guidelines to maintain effectiveness.