What configuration file is essential for defining a Liferay portlet behavior?

The correct choice identifies the liferay-portlet.xml file as essential for defining a Liferay portlet's behavior. This configuration file plays a critical role within the Liferay framework because it allows developers to specify various attributes and settings that govern how the portlet behaves and integrates within the portal.

For instance, liferay-portlet.xml can define portlet-specific settings such as its display name, the portlet's ID, the portlet class, supported modes (like view, edit, and help), and the portlet's preferences. Additionally, it serves as a bridge for the Liferay platform to recognize the portlet and manage its lifecycle effectively.

The other configuration files mentioned serve different purposes within the Liferay environment. For example, the portal.properties file is used to configure global portal settings, while portal-ext.properties allows for overriding default settings. The web.xml file is a standard Java EE deployment descriptor, which includes servlet configuration and security settings but does not specifically cater to Liferay's portlet behaviors. Hence, liferay-portlet.xml stands out as the crucial file for defining the specific behavior of a portlet within Liferay.