What advance in technology listed below has most directly helped to keep children free from disease?

The correct answer is vaccines.

Vaccines have been one of the most important medical advances in history, and they have saved millions of lives. They work by stimulating the body's immune system to produce antibodies against a specific disease, so that if the person is ever exposed to the disease in the future, their immune system will be able to quickly and effectively fight it off. Vaccines have been responsible for the eradication of smallpox, and they have also greatly reduced the incidence of many other diseases, such as polio, measles, mumps, and rubella.