Which pattern defines a one-to-many dependency between objects?

The Observer Pattern defines a one-to-many dependency between objects, allowing one object (the subject) to notify multiple dependent objects (the observers) about changes in its state. This pattern is particularly useful in scenarios where a change in one part of the system should trigger updates in multiple other parts. For example, a user interface may need to update several components when a specific data model changes.

In the Observer Pattern, the subject maintains a list of observers that have registered their interest in being notified. When there is a change in the state of the subject, it iterates through its list of observers and calls their update method, informing them of the state change. This loose coupling between the subject and observers enhances flexibility and scalability, making it easy to add or remove observers without modifying the subject's code.

The other options represent different design patterns that serve distinct purposes, such as managing interactions between components (Mediator), encapsulating algorithms (Strategy), or managing state transitions (State). However, they do not establish the same kind of one-to-many relationship that is central to the Observer Pattern.