The ability to add web servers dynamically during peak user activity and remove them when user activity decreases is an example of:

Configuration automation manages the desired state and settings of existing systems (e.g., via Ansible or Puppet) but does not involve dynamically creating or destroying server instances.

Build automation covers compiling, testing, and packaging software artifacts in a pipeline, not provisioning or deprovisioning runtime infrastructure capacity.

Infrastructure automation encompasses the dynamic provisioning and deprovisioning of compute resources - such as web servers - based on real-time demand signals, commonly implemented via auto-scaling groups in cloud or virtualized environments. This operates at the infrastructure layer itself, distinct from configuring software on existing systems or building application artifacts.

Inventory management is the process of discovering and cataloging existing IT assets, not dynamically adding or removing servers in response to load changes.