Which Kotlin feature allows extending existing classes without modifying them?

The feature that allows extending existing classes without modifying them is extension functions. In Kotlin, extension functions enable developers to add new functionality to a class without altering its source code. This is particularly useful for enhancing functionality in libraries or classes that you do not own or cannot modify.

When you define an extension function, you specify the class you want to extend and then create a new function as if it is part of that class. This approach keeps the original class intact, allowing you to enhance its functionality in a clean and maintainable way.

For example, you can create an extension function for a class like String to add new behavior, such as reversing the string or formatting it in a specific way, while the original String class remains unchanged. This promotes greater flexibility and modularity in programming, as you can customize classes to fit your needs without the risk of breaking existing code.

This makes extension functions a powerful feature in Kotlin, emphasizing the language's focus on concise and expressive code. Other choices, such as data classes or sealed classes, serve different purposes, like managing data immutability or defining a restricted class hierarchy but do not provide the capability to add new functionality to existing classes dynamically.