Which of the following was a cause of imperialism
a. people had a desire for access to raw materials and natural resources.
b. mexico became aggressive toward the united states.
c. advanced nations felt that each ethnic group should be able to pick its own form of government.
d. the united states needed more land for the growing population.

It is A but there was also 5 main causes of imperialism  

1. Political (nationalism, to be the most powerful country by ruling many other countries). 
2. Economical (to use other country's resources, recieve cheap labor, have a market for their goods. i.e. Africa). 
3. Religious (to spread their religion. i.e. missionaries) 
4. Idealogical (believed that their race was better and had to civilize other people by teaching them. i.e. The White Man's burden) 
5. Exploratory (people wanted to explore new lands for mapping, etc.)