In Adobe Commerce and Magento Open Source, the event-observer pattern is implemented through XML configuration. To subscribe an observer to a specific event, a developer must declare it within the module's etc/events.xml file. This file maps the event name (e.g., customerregistersuccess) to a specific observer class. The configuration specifies the observer's unique name and the class instance that contains the logic to be executed when the event is dispatched by the system. This file can be placed in global (etc/), frontend (etc/frontend/), or admin (etc/adminhtml/) scopes depending on the event's area.

B. Declare in etc/events.xml: This is a duplicate of the correct answer and is therefore also correct. However, in a standard multiple-choice format, only one option is typically selected.

C. Declare in etc/observer.xml: This is incorrect because etc/observer.xml is not a standard configuration file used in Adobe Commerce for declaring observers. The correct and required file name is events.xml.

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1. Adobe Commerce Developer Documentation

Events and observers: "You declare an observer in the events.xml file. The file can be in the /etc

/etc/frontend

or /etc/adminhtml directory." The documentation provides a specific example for the customerregistersuccess event.

Source: Adobe Commerce Developer Guide > PHP Developer Guide > The Core > Events and observers.

2. Adobe Commerce Developer Documentation

Module configuration files: The official list of module configuration files includes events.xml for "Event configuration" and does not list an observer.xml file.

Source: Adobe Commerce Developer Guide > PHP Developer Guide > The Core > Modules > Module configuration files.