Answer:
Stratified Random Sampling
Explanation:
Sampling is the technique of selectively choosing some units from population , for conducting a research study.
School librarian conducting research on 'books popularity among students in school' is also using sampling technique. Stratified Random sampling can be best suitable method for drawing sample students.
Random Sampling is the technique in which all units have equal chance of being chosen as sample. Stratified random sampling involves dividing the population into groups called stratas, taking sample from each strata. This makes sample a good representative of population.
In given case : Population (all school students) should be divided into class wise strata groups. Then sample should be drawn from each strata randomly. This would make students sample data a very good representative of school students population, give good results of the 'popular book' research study.