Simple sampling - A method where individuals are chosen randomly from a larger set.
Convenience sampling - The sample is being chosen as per ease of access thus it is not a random sampling.
Systematic sampling - Probability method where elements are chosen from a target population.
Stratified sampling - Method where population were divided into different groups called strata and the sample is then drawn from each group.
Cluster sampling - Population is where population were divided into different groups known as clusters. The clusters were then chosen randomly as the samples.
Answer: D (cluster sampling) - The service centers refer as the clusters and it is chosen randomly.