Answer:
At the start of the Mexican-American War, D: Polk wanted the Texas border at the Rio Grande and Mexico's Northern Territories.
Explanation:
The “Rio Grande River” forms the border between southwest “Texas” and “Mexico”. “James K. Polk” after being elected as President had a few goals. He wanted to reduce taxes on imports and expand country towards the “Pacific Ocean”. One way to expand the country was to include “Oregon territory” into U.S. But then British and US jointly claimed this area.
Then Polk wanted to buy ‘California’ from ‘Mexican government’ without any war. But Mexico did not agree to this. Soon Polk learnt that Mexican soldiers had attacked and killed the American soldiers near the Rio Grande river. Then war was declared. In 1848, the United States and Mexico signed a ‘peace treaty’. Mexico agreed to give ‘California’ and ‘New Mexico’. Also, Mexico would identify the ‘Rio Grande’ as the southern border of Texas.