Final answer:
French military reinforcements arriving in the Ohio Valley would have made the colonies unite as it posed a threat to their own control and territorial expansion.
Step-by-step explanation:
During the French and Indian War, there was a conflict between the British and the French over control of the Ohio River Valley. When Ben Franklin mentioned the arrival of French military reinforcements in the Ohio Valley, it likely meant that the French were strengthening their presence in that region. This news would have made the colonies unite because it posed a direct threat to their own control and territorial expansion. The colonies realized that they needed to join forces in order to protect their interests and defend against the French aggression.