After the Plan of San Diego true purpose was unveiled, what reality was understood by U.S. officials?

The correct understanding of the situation that arose after the unveiling of the true purpose of the Plan of San Diego is that U.S. officials realized the presence of a substantial Mexican army, which had operatives and sympathizers on both sides of the border. This understanding stemmed from fears of a coordinated uprising that aimed to reclaim territories taken during the Mexican-American War and create a new Mexican nation that included parts of the southwestern United States.

Recognizing the size and support of the Mexican army was crucial for U.S. officials as it highlighted the potential seriousness of the threats posed by revolutionary groups. It underscored the volatility of the region and indicated that this conflict was not merely a small-scale local insurrection but rather part of a broader revolutionary wave that could destabilize both Mexico and the southwestern U.S.

The other choices do not accurately reflect the broader geopolitical context or the insights gained by U.S. officials at the time. While the plan's implications involved serious concerns, they were rooted in the actual military dynamics rather than outlandish claims or simply economic effects.