Answer: Cluster sampling.
Explanation:
- Cluster sampling is a sampling technique in statistics in which the researcher splits the entire population into different groups called clusters.
- After that he randomly select a sample from the clusters from the population and survey all elements of sampled clusters.
- Researcher performs his analysis on data from the sampled clusters.
In the given situation Greyhound Lines randomly selects 90 buses during a certain week and surveys all passengers on the buses.
Here, week= Cluster
Buses = Elements
Therefore , the type of sampling is used = Cluster sampling.