The correct answer is - a. Because of the changed border, many Mexicans became Americans.
After the Mexican- American War the demographics of the United States changed because of the inclusion of the new territories in its own territory. The new territories had a significant number of Mexican population in them, but the Mexican did not tried to fled the country and go to Mexico, nor the United States expelled them from the country, so the total population of the United States changed in a very short period of time, as well as the ethic structure of the country. Even though the Mexicans wanted this territories to be reunited with Mexico still the American authorities managed to integrate them into the society and in time partially assimilate them to the point where they started feeling like Americans.