Final answer:
The best title for the textbox is 'c. Causes Leading to the Annexation of Texas,' which reflects the various factors that led to Texas's incorporation into the United States, such as American settlers' desire for expansion and Texas's own financial problems.
Step-by-step explanation:
The best title for the textbox that discusses American Texans' desire to become part of the United States, the United States' desire for Manifest Destiny, and the Republic of Texas's financial problems would be c. Causes Leading to the Annexation of Texas. This title accurately encapsulates the combination of economic, ideological, and political factors that drove the eventual decision to incorporate Texas into the United States. The concepts of Manifest Destiny and the financial struggles of the Lone Star Republic were significant in propelling the annexation movement, while the shared ambition of settlers in Texas and expansionists in the United States merged, pushing towards the inevitable inclusion of Texas into the Union.
The annexation of Texas was a contentious issue, entwined with the political strife over slavery and the balance of power within the United States. The reluctance to admit Texas as a slave state stemmed from the potential to incite conflict with Mexico and the internal discord over the spread of slavery, illuminating the complexities of national politics preceding the Mexican-American War and exacerbating tensions leading up to the Civil War.