Microsoft Certified Solutions Developer (MCSD) Certification Practice Test

Extension methods in C# are a powerful feature that allows developers to add new methods to existing types without modifying their source code. This is particularly useful for enhancing functionality in a way that promotes cleaner code especially in cases where you do not have access to the original class or when you want to keep the original type unchanged.

By using extension methods, you can add functionality to classes such as `IEnumerable<T>` or even to your own custom classes, making them more versatile and easier to work with. These methods are defined as static methods in a static class, with the first parameter specifying the type they are extending, prefixed with the `this` keyword. This allows the method to be called as if it were an instance method on the extended type.

For example, if you create an extension method for `string`, once the extension is defined, you can call the method directly on any string object, enhancing readability and maintainability of the code.

The other options do not accurately describe extension methods. Extension methods do not modify the original class or base class, cannot be defined just in a new namespace without the static class context, and while they are defined as static methods, the static nature alone does not capture the concept of extending existing types.