Which software model is characterized by a combination of different functionality from various sources?

The correct choice is characterized by the integration of multiple functionalities from diverse sources, creating new applications or tools. Mashups are a specific type of software that utilizes APIs and data from different websites and online services to provide a unique service or output. This allows for seamless user experiences by combining features and data into a single interface, often enhancing the original functionalities of the individual components.

Other options represent distinct software concepts. Vertical-market applications refer to software designed for a specific industry or sector, focusing on meeting specialized needs which limits their scope to particular functionalities rather than combining various sources. Web applications are designed to run on web servers and accessed through browsers; while they may integrate with APIs, they do not inherently imply the combination of different functionalities from multiple sources as mashups do. Public-domain software refers to software that is freely available for use without restrictions, but it does not inherently describe the integration of diverse functionalities.