The basal regions of the upright human lung are normally better ventilated than the upper regions because:

The lower regions of the lung have a relatively small resting volume and large increase in volume compared with those near the top of the lung. The other choices are incorrect: The airway resistance of the upper regions is probably somewhat less than that of the lower regions because the parenchyma is better expanded there. However, in any event, this is not the explanation of the difference in ventilation. There is no evidence that there is less surfactant in the upper regions of the lung. It is true that the blood flow to the lower regions is higher than to the upper regions, but this is not relevant here.