Final answer:
The Mexican War began in the year 1846.
Therefore, the correct answer is: option a. 1846
Step-by-step explanation:
President James K. Polk, driven by the desire for territorial expansion to the Pacific Ocean and influenced by pro-slavery interests, saw the annexation of Texas and border disputes as opportunities for enlarging the United States.
In January 1846, American troops were sent to the Rio Grande area to enforce claims from the border dispute, leading to clashes with Mexican forces.
This resulted in President Polk asking Congress to declare war on May 11, 1846, and by September 1847, American troops had invaded Mexico City.
This conflict, known as the Mexican-American War, eventually concluded with the Treaty of Guadalupe Hidalgo in 1848, whereby Mexico lost approximately half of its territory to the United States.