What are Kotlin extension functions?

Kotlin extension functions are a powerful feature that enables developers to augment existing classes with additional functionality without the need to alter their source code. This is particularly beneficial when dealing with classes from third-party libraries or standard libraries where you cannot and often do not want to change the original implementation. By using extension functions, you can create new functions that appear to be part of the class, allowing for more expressive and clearer code.

For instance, if you have a class that represents a String and you want to add a method that checks if the string is a palindrome, you can define an extension function for the String class. This new function will behave as if it is part of the String class, providing convenience and readability.

In summary, the correct answer highlights how extension functions enable additional functionality without necessitating changes to existing class definitions, thus embracing the principles of extensibility and code reusability in Kotlin programming.