Final answer:
A User Interface Integration (UII) action defines the action to be performed within a software system, representing an operation triggered by various events that form a significant aspect of the user experience.
Step-by-step explanation:
A User Interface Integration (UII) action in the context of software and web development refers to C. the action to be performed. UII is a framework that allows different user interface components from various applications to be integrated and work cohesively. In more detail, when we talk about a UII action, we are referring to an operation or a function that is triggered by some event or command within the system. This action could range from displaying data on the screen, submitting forms, to calling backend services based on user interactions or automated processes. As a fundamental aspect of UII, distinguishing actions is essential because they define the behavior of the system in response to the user or system events, thus forming a significant part of the overall user experience.