The correct answer is D.
The Texas Annexation into the United States of America took place in 1845. The majority of the Texian population supported it. But, on the US side, the main political parties opposed it because of the importance of slavery in that territory. Moreover, it could start a conflict with Mexico, that had not acknowledged yet the independence of their former Northern province, although it has seceded ten years before.
Therefore, it was the spark with gave rise to the Mexican-American War (1846-1848)
