Cluster sampling

A researcher studying employee wellbeing at a nationwide supermarket chain randomly selects 20 branches and surveys every employee within those branches. What sampling method is this, and what is its primary statistical limitation?

• AStratified sampling; the limitation is that branches from some regions may be entirely excluded from the sample.
• BSystematic sampling; the limitation is that the fixed selection interval may coincide with a periodic pattern in branch performance.
• CCluster sampling; the main limitation is lower precision than a simple random sample of the same total size, because employees within a branch tend to be similar to each other.
• DSimple random sampling; the limitation is that it requires a complete list of all employees, which may not be available.