The best explanation for the role of external forces in the Ming Dynasty's fall 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 response highlights how the financial and military strains due to external pressures, including conflicts with the Jurchens and other groups, contributed to the weakening of the Ming Dynasty, allowing the Manchus to eventually overthrow it.