The right answer is B. Here some explanation:
In 1845, American President James K. Polk (1845-1849), imbued with the spirit of "manifest destiny" (it was an idea that the United States had God's right to expand its territory); has shown interest in further expanding American territory. The target this time was the Mexican state of California. The US government offered the sum of $ 25 million for Mexico to sell the states of California and New Mexico. The offer was rejected by the Mexican government. In addition, in 1845, the United States had begun the Texas annexation process, which displeased the Mexican government who still claiming that territory. Finally, there was a border dispute that debated the extension of the Mexican border with Texas. In this matter, the American government affirmed that the frontier extended until the Rio Grande, whereas the Mexican government established that frontier in the Nueces River.
In April 1846, the Mexican president declared that Mexico was in a "defensive war" against the United States. This declaration began the actual hostilities between these two countries. The conflict resulting from these frictions lasted for two years, with the US military winning all major war battles. The highlight of the American victory was the conquest of Mexico City, the Mexican capital, in September 1847, after the victory at the Battle of Chapultepec.
