Which of the following changes is considered a breaking change to an existing schema?

A breaking change to an existing schema refers to alterations that disrupt the existing structure, potentially causing downstream dependencies to fail. When fields are removed from a schema, it means that any integration or application relying on those fields will no longer function as expected. This can lead to compatibility issues where existing data may not align with the new schema, and systems expecting those fields will struggle to process information correctly.

In contrast, introducing new optional fields does not disrupt existing functionality, as it allows for backward compatibility. Adding comments to the schema is purely informational and does not affect how data is processed or managed. Changing the order of fields may have implications in some contexts, but it typically doesn't break existing references or functionality since data retrieval usually accommodates such changes. Thus, the removal of previously defined fields is the only option that directly leads to a breaking change in the schema.