Vertical scaling (scale-up/down) involves adding more resources (CPU, RAM, storage) to an existing virtual machine to increase its capacity. Option A (adding an additional CPU to an existing Azure virtual machine) directly exemplifies this by enhancing the power of a single VM. In contrast, options B and C represent horizontal scaling (scale-out/in), which adds more instances (VMs or session hosts) to distribute load, and option D describes automatic horizontal scaling via Azure App Service. The community discussion, with 100% consensus and upvoted comments (e.g., TinaTina's explanation with 14 upvotes), confirms that vertical scaling focuses on boosting existing resources, not adding instances.