Which of the following is an example of vertical scaling?

Vertical scaling, also known as "scaling up," refers to the process of adding resources to a single server or system to improve its capacity and performance. When you increase the CPU power of a server, you are enhancing the server's ability to handle larger workloads, thereby increasing its performance. This directly contributes to improving the overall capacity of the server without needing to involve additional servers or distributed resources.

The other options involve strategies that focus on increasing capacity by utilizing multiple resources rather than enhancing a single resource's performance. Adding more servers is a classic example of horizontal scaling, which involves bringing in additional machines to share the workload instead of upgrading existing ones. Distributing traffic across servers also indicates horizontal scaling, as it implies multiple servers working together instead of enhancing the performance of one server. Load balancing, while essential for optimizing resource usage across various servers, does not constitute vertical scaling since it is about managing multiple servers rather than upgrading a single one.