Which of the following cannot be accomplished with a hook plugin?

A hook plugin in Liferay is primarily designed for extending and modifying the behavior of the existing core functionalities without changing the core code itself. This ability allows developers to customize Liferay's platform effectively while retaining the upgradeability of the core system.

The correct option—altering the default theme layout—cannot be accomplished with a hook plugin because hooks do not have the capability to modify frontend elements like themes directly. Instead, changes to themes, such as layout, styling, and other presentation aspects, are typically done using theme plugins. Theme plugins are specifically created for designing how the site appears, including layout alterations, which is outside the scope of what a hook can manage.

In contrast, a hook can override Struts actions to customize the backend functionality of applications within Liferay. It can also be used to intercept requests and implement custom logic, making it capable of altering core services as well. Additionally, hooks can manage custom configurations through properties files, which allows for tailored settings in the Liferay environment.

This distinction between what a hook can do in terms of the backend and restrictions related to frontend appearances places the alteration of the default theme layout firmly outside the capabilities of a hook plugin.