To gather information about the validity of a new standardized test for 10th-grade students in a particular state, a random sample of 15 high schools was selected from the state. The new test was administered to every 10th-grade student in the selected high schools. What kind of sample is this?

a. A simple random sample

b. A stratified random sample

c. A cluster sample

d. A systematic random sample

e. A voluntary response sample

Select the best answer.