Apache Kafka provides two primary mechanisms for data retention, configured via the cleanup.policy topic-level setting. The delete policy, which is the default, discards entire log segments after a configured retention period (time-based) or size limit is reached. This is suitable for transient data. The compact policy enables log compaction, which retains at least the last known value for each message key within a partition. This is ideal for use cases requiring a full snapshot of the latest state, such as restoring a database. It is also possible to set the policy to compact,delete to both compact the log and then delete records older than a certain age.

B. default is not a valid value for the cleanup.policy configuration. The policy that is used by default is delete.

D. cleanup is part of the configuration parameter's name (cleanup.policy), not a valid value for the policy itself.

1. Confluent Documentation: In the "Topic-Level Configs" section, the cleanup.policy parameter is defined. It explicitly states, "A string that is either delete, compact, or both (compact,delete). This string designates the retention policy to use on old log segments."

Source: Confluent Documentation, "Server Configuration Properties," Section: cleanup.policy.

2. Apache Kafka Documentation: The official Apache Kafka documentation corroborates this in its "Topic Configs" section. It specifies the valid values for cleanup.policy are "delete, compact".

Source: Apache Kafka Documentation, "3.7 Configuration," Section: "Topic Configs," Parameter: cleanup.policy.

3. Kleppmann, M. (2017). Designing Data-Intensive Applications. O'Reilly Media. Chapter 11 discusses log-structured storage and messaging systems. It details the concepts of log compaction (as used in Kafka's compact policy) and time-based retention (as used in the delete policy) as fundamental strategies for managing log data.

Source: Chapter 11, "Stream Processing," Section: "Messaging Systems." (Note: While a book, it is a peer-reviewed and highly respected academic-level text in the field).