What UML diagram is commonly used during the testing phase to illustrate interactions?

The Sequence Diagram is the correct choice because it is specifically designed to illustrate how objects interact in a particular scenario of a use case, detailing the sequence of messages exchanged between them over time. This is particularly important during the testing phase, where understanding the flow of interactions can help in identifying whether the system behaves as expected in response to various inputs.

In a Sequence Diagram, the focus is on the order of messages and the timing of those messages, which can effectively highlight any discrepancies or issues in the interactions being tested. This makes it a valuable tool for testers who need to verify that the interactions among components work according to the specifications laid out in the design phase.

Other diagrams, while useful in their own contexts, serve different purposes: Class Diagrams represent the structure of the system by showing classes and their relationships, Component Diagrams depict the organization and dependencies among a set of components, and Use Case Diagrams outline the functionality of the system from a user's perspective but do not detail the specific interactions. Therefore, the Sequence Diagram stands out as the best option for illustrating interactions during testing.