A. Answering parents' questions about the safety and importance of vaccines today: While educating parents is essential, it relies on voluntary compliance and does not have the same large-scale impact as mandatory immunization laws. B. Offering all immunizations to all children for a small fee: Improving vaccine access is important but does not guarantee that parents will vaccinate their children. C. Educational campaigns to all health care providers about the importance of immunizations whenever a child is seen: While provider education is valuable, it does not enforce vaccinations on a broad population level like school-entry laws do. D. "No shots, no school" legislation, which legally requires children be immunized before school: School immunization mandates have played a significant role in increasing vaccination rates and reducing outbreaks of preventable diseases such as measles, mumps, and pertussis. These laws ensure high vaccine compliance before children enter school, making them one of the most effective public health measures.