The response that best explains the role of external forces in the fall of the Ming Dynasty is:
The expense of fighting external forces weakened the empire as new groups, such as the Jurchen (later known as the Manchus), gained control.
This statement highlights how the financial and military strain from external conflicts, particularly with groups like the Jurchens, contributed to the decline of the Ming Dynasty and ultimately facilitated the rise of the Manchu-led Qing Dynasty.