The option that best explains the role of external forces in the Ming Dynasty's fall is:
Economic collapse weakened the Ming Dynasty due to the expense of protecting its borders just as the Jurchen (Manchus) gained power.
This option highlights how external pressures, particularly from the Jurchens (Manchus), combined with the internal economic struggles, contributed significantly to the Ming's downfall. The costs associated with defending against external threats strained the dynasty, making it vulnerable to the rising power of the Manchus.