Answer
Cluster sampling. See explanation below.
Step-by-step explanation:
For this case they not use random sampling since we are selecting people from flights. Because we select just 5 random flights.
Is not stratified sampling since we don't have strata clearly defined on this case, and other important thing is that in order to apply this method we need homogeneous strata groups and that's not satisfied on this case.
Is not convenience sampling because they NOT use a non probability method in order to select the people from the flights.
So then the only possible method is cluster sampling since we have clusters clearly defined (Passengers from the airlines), and we satisfy the condition of homogeneous characteristics on the clusters and an equal chance of being a part of the sample, since we are selecting RANDOMLY, the 5 flights to take the information.
