Which operationally efficient solution meets these requirements?

Explanations:

Using Lambda functions in each member account adds significant operational overhead for management and maintenance, especially as the number of accounts grows. This approach is complex and not the most efficient way to centralize CloudTrail logs. Setting up CloudWatch alarms is a good practice, but this doesn’t solve the core problem efficiently.

While delivering logs to a central S3 bucket is the right idea, configuring CloudTrail individually in each member account and managing individual bucket policies is not scalable. This also does not automatically apply to new accounts. Migrating existing logs is a necessary step but is not the complete solution.

Configuring anorganization-levelCloudTrail in the parent account is the most efficient solution. This automatically configures CloudTrail logging forallexisting andfuturemember accounts, minimizing administrative overhead. Migrating existing logs ensures that no data is lost. Deleting the old CloudTrail configurations and logs in member accounts after migration is a good practice to avoid duplication and confusion.

Configuring CloudTrail inboththe parent account at the organization levelandindividually in each member account would result in duplicate logs being sent to the central S3 bucket. This creates unnecessary storage costs and complexity.