Answer:
D. Annexation of Texas.
Explanation:
Remember from the previous answer, the Manifest Destiny was the "justifiable source" in the expansion of the US westward. What is not known as Texas, was part of the Manifest Destiny. During this time, the Mexican Government allowed for US citizens to populate the lands that stretch from Texas to parts of California. However, it is important to note that they did not allow slavery, which is a large factor in the Texas Independence. With the independence of Texas, and the annexation of Texas from the Republic of Texas to the state of Texas, the US added a large sizable chunk to it's borders, further expanding the US in the southwest sector.
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