Design a Pub/Sub push-subscription system for clickstream where subscribers may be temporarily down. Requirements: store unconsumed input, retry failures gradually to avoid overload, and after max 10 retries store failed messages in a topic. How configure the subscription?

The correct option uses exponential backoff for the retry policy and dead lettering to a different topic. Exponential backoff is crucial for gradually retrying failed messages, preventing subscriber overload, and aligning with the requirement to retry failures gradually. Dead lettering to a different topic ensures that messages that ultimately fail after 10 retries are isolated for further analysis without polluting the original source topic or getting stuck in a retry loop. The maximum delivery attempts set to 10 directly addresses the requirement for a maximum of 10 retries before moving to the dead-letter topic. Increasing the acknowledgement deadline only delays re-delivery, it doesn't implement a retry strategy or handle eventual failures. Using immediate redelivery would overwhelm a temporarily down subscriber, failing the "retry gradually" requirement. Dead lettering to the same source topic would create a circular dependency and make it difficult to distinguish between new messages and permanently failed messages.