The correct answer is D) craftsmanship.
The advancement that would have been directly responsible for the army that protected the city of Thebes is craftsmanship.
Some historians refer to The Sacred Band of Thebes as an elite army unit that were lovers. They were 300 soldiers. Historians that have read texts of that epoch consider that the band was a solid group because that the group was united by love and that made them unbreakable and unbeatable. This kind of unity and support for each other was the reason than Thebas had an important place in the 4th century. The advancement that would have been directly responsible for the army that protected the city of Thebes is craftsmanship.