Answer:
E. A stratified random sample
Step-by-step explanation:
Previous definitions
A census "is an official survey of the population of a country that is carried out in order to find out how many people live there and to obtain details of such things as people's ages and jobs". Is important to notice that in a census we survey all the people in a population and thats not our case since we are using sampling. So this one is not the correct option.
Cluster sample is a" techinique on the researcher divides the population into separate groups, called clusters. Then, a simple random sample of clusters is selected from the population. The researcher conducts his analysis on data from the sampled clusters". For this case we don't have clusters (we dont' have natural groups separating the population) since we don't separate the population in all the possible public and privates schools.
Simple random sample is a "techinique that take a subset of a statistical population in which each member of the subset has an equal probability of being chosen". On this case that's not the correct option since the researcher select 20 private and 20 public schools, and we don't have probability of inclusion for the other possible public and private schools.
Stratified random sample is a "method of sampling that involves the division of a population into smaller groups known as strata. In stratified random sampling or stratification, the strata are formed based on members' shared attributes or characteristics".And for this case is the best option we have the strata defined and we apply the sampling method on these strata.
