Why did some members of Congress express doubt about going to war with Mexico? a) The territory where “American blood was shed on American soil” was actually
disputed territory
b) They were not sure if we would be able to defeat the Mexican army.
c) They were not willing to spend the money fighting Mexico over land we
already owned.
d) They didn't want to allow another slave state into the country.

d) They didn't want to allow another slave state into the country.

Explanation:

The United States went to war against Mexico over the issue of Texas independence from Mexico, and subsequent annexation to the United States, a position that was supported by most of its inhabitants (including those of Mexican descent, also known as Tejanos).

However, the problem was that if Texas was admitted to the United States, it would become a slave state. The Eastern Part of Texas has the same climate and geography as Louisiana or Mississippi, and therefore, large slave plantations could be started there, and the annexation of Texas would then mean that the delicate balance between Free States and Slave States would break in favor of the slaves states. This is essentially what made Congress doubt about going to war with Mexico.