Answer: D) all American school teachers.
Explanation:
In statistics, A population is a very large set of individuals or values that belong to a specified group established by researcher.
A sample is a subset of population that represents the entire population in research. 
Given : We want to know the opinions of American school teachers about establishing a national test for high school graduation. 
Clearly , by the above definition
Population = all American school teachers.
 You obtain a list of the members of the National Education Association and mail a questionnaire to 2500 teachers chosen at random from this list.
sample = 2500 teachers chosen at random from this list. 
Hence, the intended population is all American school teachers.
Thus , the correct answer is D) all American school teachers.