The correct answer to this open question is the following.
"Students are selected at random from lists of students at a university organized by class level such that the proportion of individuals at each class level in the population of students matches the proportion selected for the sample. The type of sampling that is being utilized is a stratified random sample.
This is a type of probability sample that is used when we want to divide the total population into multiple, non-overlapping groups that are homogeneous. Then we want to choose randomly final members of the homogeneous group to do research. Members have to be distinct in order to get equal opportunity to be chosen in a simple probability. The stratified random sample uses classifications such as nationality, socioeconomic differences, religion, age, and education, among many others.