Final answer:
The best selection showing opposition to U.S annexation of Texas is the concern that it would set a precedent for further U.S territorial expansions, threatening regional stability.
Step-by-step explanation:
The selection that best shows why other nations oppose the U.S annexation of Texas is "Opposing nations worry that the annexation of Texas would set a precedent for further territorial acquisitions by the United States."
This statement encapsulates the concerns of other nations that the addition of Texas to the United States would not be an isolated event but rather the beginning of a pattern of territorial expansion that might threaten the balance of power in the region. It implies that annexation could lead to further American territorial ambitions, affecting neighboring countries and potentially destabilizing the status quo.