What is commonly used to develop applications for iOS devices?

The most commonly used tools for developing applications for iOS devices are Xcode and Swift. Xcode is Apple's official integrated development environment (IDE), which provides developers with the necessary tools to create software for iOS, macOS, watchOS, and tvOS. It includes a graphical user interface (GUI) for building app interfaces, debugging tools, and performance testing options.

Swift is a programming language developed by Apple specifically for building iOS and other Apple platform applications. It is designed to be easy to read and write, while also being powerful enough to create complex applications. The combination of Xcode and Swift is highly favored in the development community due to their seamless integration, robust features, and extensive documentation provided by Apple.

In contrast, the other options represent tools and languages primarily associated with different platforms or environments. Android Studio and Java are used for Android app development, Eclipse and C++ can be utilized for various programming projects but are not specifically aimed at iOS development, and Visual Studio along with C# is typically associated with Windows and cross-platform app development rather than native iOS applications.