A national survey of middle-school students asks which subject is most challenging. Which of these samples is a representative sample.

a) 372 sixth-graders in a certain town

b) 972 seventh-graders in a certain country

c) 619 eight-graders in different states

d) 400 students from various states, representing different grades, 6-8

The correct option is D.

Step-by-step explanation:

For a research sample to be described as representative, the sample must be a subset of the target population group and it must accurately reflects the features of the target population group.

In this question, the target population is the middle school students of a country, that is, grades 6 to 8. The survey is a national one, that is, it covers the whole country. In order to obtain a representative samples for this particular survey, samples must be drawn from grade 6 to 8 and the samples must come from different parts of the country. This is why option D is correct.

The other options are not correct because, the samples are limited to only particular grades or particular towns or states.