Final answer:
Cross-region replication in Amazon S3 starts immediately when you enable replication rules; it affects objects uploaded or modified after the rule is set.
Step-by-step explanation:
Cross-region replication (CRR) in Amazon S3 begins almost immediately after the replication rule is added and enabled. This means that the correct answer to your question is (A) Immediately upon enabling replication. Once replication is enabled, any new objects uploaded to the source bucket or any existing objects that are updated will trigger the replication process. However, it's important to note that only the objects created or updated after the rule is enabled will be replicated. Previous objects will not be affected unless they are modified after the replication is set up. To replicate existing objects, you would typically have to copy them manually or using a script to simulate an update so that CRR starts for those objects.