Socket.io is a popular library used in conjunction with React for managing Websocket connections. Socket.io simplifies real-time communication by providing a WebSocket API that works seamlessly with React applications. Redux, React Router, and Axios are important libraries, but they are not specifically designed for Websockets; they serve other purposes, such as state management, routing, and HTTP requests, respectively.

Error boundaries in React are primarily used to catch and handle errors gracefully, preventing them from crashing the entire application. When an error occurs within the boundary of an error boundary component, React can display a fallback UI or perform other actions to ensure a smoother user experience. They don't prevent all errors but provide a way to manage them more gracefully.

The pattern where only data changes (deltas) are sent instead of the full dataset is known as "Delta Streaming." In real-time financial data scenarios, sending only the changes in data helps optimize bandwidth usage and improve the overall performance of the application. It reduces the amount of data transmitted and processed, making it a common practice in real-time applications.

Excessive re-renders in a React component can often be caused by not using PureComponent or memoization. PureComponent and memoization techniques help prevent unnecessary re-renders when no new data has arrived, optimizing performance. Addressing this issue involves implementing PureComponent or memoization in the component to reduce unnecessary rendering. The other options do not directly address the probable cause.

When using React's React.lazy() for code splitting and dynamic imports, it's recommended to handle network failures by wrapping your lazy-loaded components with an ErrorBoundary. The ErrorBoundary component catches any errors that occur during the loading of the lazy component and allows you to display a fallback UI or handle the error gracefully.

To create a cross-platform mobile application using React's principles, developers often turn to React Native. React Native is a framework that allows developers to build mobile applications for iOS and Android using React's component-based approach. Angular, Vue.js, and Xamarin are alternatives to React Native for cross-platform mobile development, but they are not specifically built on React's principles.

When using the Context API, wrapping components with the "" component allows them to consume context values. The "" component provides a way for components within the tree to access and use context data provided by a parent "" component. This mechanism is fundamental for passing and retrieving context values in React applications.

In Redux, the function responsible for specifying how the state should change in response to dispatched actions is called a "reducer." A reducer takes the current state and an action as input and returns the new state based on the action type. It's a fundamental concept in Redux for managing state changes in a predictable manner.