The correct answer is A) they developed an extensive system of paved stone roads.
The factor that allowed the Incas to use their military efficiently was that they developed an extensive system of paved stone roads.
The Incas lived in the Andean Mountains of Peru. In their best years, they had an impressive army that allowed them to defend their territory from other tribe's invasions. One of the advantages of the Inca army was indeed the well-designed system of roads that allowed fast transportation of warriors and supplies to many different parts.
