Which of the following is an example of horizontal scaling?

Migrating a VM from one host to another is a live migration or workload balancing operation - it moves existing capacity but does not add new capacity to the environment.

Adding VM guests to a host increases utilization of the existing host's resources but does not introduce new capacity into the cluster, so it is not a scaling operation.

Adding CPU to a host is vertical scaling (scale-up) because it increases the resource capacity of an existing node rather than adding a new node to the infrastructure.

Adding a host to a cluster is horizontal scaling (scale-out) because it increases total capacity by introducing a new independent node rather than upgrading existing hardware. Workloads can then be distributed across more hosts, improving both capacity and fault tolerance without modifying any existing machine.