A null hypothesis regarding the incidence of childhood caries could read

The null hypothesis serves as a foundational statement in hypothesis testing that suggests there is no effect or no difference between groups in question. In the context of this scenario, the null hypothesis indicates that there is no significant difference in the oral health between rural and city children. Therefore, stating that the oral health of the rural children will not differ from that of the city children perfectly encapsulates this idea.

A null hypothesis is typically framed in a way that asserts that any observed difference in data is due to sampling variability or random chance rather than a true effect. The formulation that rural children's oral health will not differ from that of city children aligns with this principle, as it posits a baseline condition where any differences observed in a study would be negligible or attributable to randomness rather than a systematic issue.

In contrast, the other statements propose specific directional differences between the groups, indicating that one group's oral health is significantly worse or better than the other. By making these assertions, they imply a difference in health conditions, which contradicts the essence of a null hypothesis and suggests an alternative hypothesis instead. Thus, the correct answer effectively captures the intention of a null hypothesis within the context of childhood caries incidence.