Why did Americans find the Zimmermann note so threatening?

An act of war

Explanation:

The U.S. was not interested in war, however with the greatly increasing amount of America ships sunk in unrestricted submarine warfare, The U.S. was pushing towards war. The British intercepted the Zimmerman note that the Germans hoped the Mexicans received. This note was very threatening to the U.S. since it called for Mexico to invade America, including Texas and its borders. The U.S. most definitely didn't want a war on their own soil, against a neighbor.

a, c, & d are correct:

they wanted lands to settle the extra population.

they wanted new markets for english goods.

they wanted to avoid civil war.

b. second continental congress