Explanation:

When you create custom modules using the XML SDK in Mule 4 (for example, reusable components or connectors), testing them requires a special setup. While MUnit is the core testing framework in MuleSoft, testing XML SDK modules specifically requires the:

🔧 MUnit Extensions Maven Plugin

This plugin enables:
➞ Unit testing of custom components built with XML SDK
➞ Generation and execution of tests for extensions and custom modules
➞ Integration of your module tests into Maven’s test lifecycle

❌ Why the other options are incorrect:

A. XMLUnit
🔸 Incorrect: This is a general-purpose Java/XML testing library, not specific to Mule or XML SDK.

B. JUnit
🔸 Incorrect: JUnit is the base unit testing framework (used internally by MUnit), but it doesn't support Mule-specific or XML SDK-specific needs out of the box.

D. MUnit Maven Plugin
🔸 Incorrect: This plugin is used for testing Mule applications, not XML SDK modules. It works with .xml flows, not extension modules.

🔗 Reference:
MuleSoft XML SDK Testing Guide
MUnit Extensions Plugin GitHub