The correct answer is - True.
California was part of Mexico until approximately century and a half ago. Mexico had much larger territory then what it has nowadays. The western, southwestern, and westernmost part of the Great Plains in what is now the United States, were all part of Mexico. It can easily been seen even nowadays that that was the case in the names of the settlements, rivers, mountains...
Mexico though had a territory that it was not powerful enough to control and protect once the Spanish power and support were gone. The United States, on the other side, had expansionist politics, it was a growing power, and once it saw a god opportunity it challenged Mexico. Mexico was not able to defend its territories and lost them, with the United States incorporation large portion of its territory.
