The statement that BEST describes the efforts made by the United States to annex Texas and the Mexican Cession is "The need to fulfill Manifest Destiny."
This is evident because when the United States wanted to annex Texas to the union, there were mixed reactions; some supported the action, while some were against it.
However, many people who supported Texas's annexation and the Mexican Cession believed that through Manifest Destiny, the United States must spread from the Atlantic to the Pacific Ocean.
Option A is wrong because the United States had to engage in war with Mexico to annex Texas.
Option B is wrong because Texas was annexed into the Union as a Slave state.
Option D is wrong because Europeans are not involved in the issue.
Hence, in this case, it is concluded that the correct answer is Option D.
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