Which framework is used in Liferay for creating custom portlets?

The OSGi Framework is the correct answer as it is fundamental to how Liferay manages and deploys its modular components, including custom portlets. OSGi (Open Services Gateway initiative) provides a dynamic module system for Java that enables the development of modular applications, which is essential in a portal environment like Liferay where applications need to be able to be added, updated, or removed without requiring a full redeployment.

In Liferay, each custom portlet is packaged as an OSGi module. This modular architecture allows for better separation of concerns, improved dependency management, and the ability to deploy individual features or portlets independently of the entire application. Liferay's reliance on OSGi facilitates runtime service discovery and binding, which enhances flexibility in how developers create and manage portlet applications.

Other frameworks mentioned, while popular in their own domains, are not specifically utilized for creating custom portlets in Liferay. The Spring Framework is often used for building enterprise applications but does not provide the modularity that OSGi does in the context of Liferay. Angular and React are JavaScript frameworks primarily used for building user interfaces rather than back-end integrations and modular services that align with OSGi's purpose within the Liferay ecosystem.