For a female patient with suspected spinal injury and reduced tidal volume, the most appropriate airway management is:

In cases of suspected spinal injury coupled with reduced tidal volume, it is crucial to ensure adequate ventilation for the patient. Assisting ventilations at an age-appropriate rate allows for the delivery of effective breaths that meet the patient's specific respiratory needs, taking into consideration her age and physiological factors. This approach not only helps in stabilizing the patient’s condition by improving respiratory function but also minimizes the risk of further injury that might occur through excessive ventilation or improper techniques.

This choice stands out as the most appropriate because it balances the need for effective ventilation with the potential risks associated with inappropriate breathing rates. The other options could lead to either hyperventilation or inadequate oxygenation, compromising the patient's health further. Therefore, assisting ventilations in a manner appropriate for her age is the best practice to optimize breathing and oxygen delivery without exacerbating her condition.