What is a requirement for synchronization of unsubscriptions between different message senders?

The correct requirement for synchronization of unsubscriptions between different message senders is that it needs integration with the SOAP API. The SOAP API provides a standardized way to interact with various components of the Salesforce Marketing Cloud. Specifically, it allows for seamless communication between different systems, ensuring that when a subscriber opts out of communications, this change is propagated across all message sending platforms.

Utilizing the SOAP API allows organizations to manage subscriber preferences effectively, ensuring that unsubscription requests are uniformly updated across all related systems. This integration ensures that any opt-out action is recognized and applied universally, preventing any further messages from being sent to individuals who no longer wish to receive them.

The other options, while they might relate to data management or marketing processes in their own right, do not directly address the technical requirement for synchronizing unsubscriptions across different message senders as effectively as the SOAP API does.