What can a hook plugin extend and override?

A hook plugin in Liferay is specifically designed to extend and override functionalities of the core platform, which includes core portal Struts actions. When a hook is implemented, it can modify the behavior of these actions, enabling developers to customize how the portal operates without altering the original source code directly. This capability is crucial for maintaining upgradability and stability, allowing organizations to adapt Liferay to their specific business needs while leveraging the robustness of the core platform.

The other options, while relevant to Liferay development, do not encapsulate the primary purpose of hook plugins. For example, third-party plugins, layouts and themes, and web content templates can indeed be customized in various ways, but they are not the main target for hooks, which are primarily focused on the underlying actions and processes of the portal itself.