Which of the following is a popular open-source SDN controller?

OpenDaylight is recognized as a popular open-source software-defined networking (SDN) controller because it was specifically designed to support the development and deployment of SDN solutions. It provides a robust framework that enables network operators to manage their infrastructure in a programmable manner, allowing for greater flexibility, automation, and innovation in networking.

OpenDaylight acts as a platform that can integrate various networking protocols, enabling a wide range of applications and services that can be tailored to specific network requirements. Its modular architecture supports different northbound and southbound interfaces, making it adaptable to various network environments and use cases. This adaptability, along with strong community support and contributions from a diverse set of organizations, has solidified OpenDaylight's place as a leading solution in the realm of SDN controllers.

In contrast, the other options listed serve different purposes: OpenStack is primarily a platform for managing cloud resources, OpenShift is an enterprise Kubernetes platform focused on container orchestration, and Kubernetes is itself a system for automating the deployment, scaling, and management of containerized applications. While they all play significant roles in cloud and network management, they do not specifically function as SDN controllers like OpenDaylight does.