Before the Mexican-American War, the correct option would be:
A. In the 1830s, Americans and some Mexicans living in Texas revolted against Mexico.
This refers to the Texas Revolution, which occurred from 1835 to 1836, when Texian settlers (many of whom were American immigrants) revolted against the Mexican government, leading to the establishment of the Republic of Texas. The other options did not happen in the timeframe leading directly up to the Mexican-American War.