HTTP 429 errors indicate throttling (“Too Many Requests”). In this architecture, throttling can occur at multiple layers (API Gateway rate limits, downstream AWS service API throttles, or dependency throttling). Regardless of which service is throttling, the correct resilience pattern is to implement retries with exponential backoff and jitter for retryable failures. AWS SDKs and AWS best practices recommend backoff to reduce contention and allow the system to recover, while ensuring requests eventually succeed.

Option C directly addresses the problem and the requirement “ensure that all of the application ingests all files.” With a retry + backoff pattern, transient throttling is handled gracefully: failed operations are retried after increasing delays, avoiding immediate retry storms that worsen throttling. This increases successful completion rate without requiring major architectural changes.

Option A (Transfer Acceleration) can improve upload latency from geographically distributed clients, but it does not resolve request throttling (429) caused by API limits.

Option B (multipart upload) helps with large object uploads and can improve throughput/reliability for big files, but it does not inherently prevent 429 throttling at API Gateway or other throttled calls.

Option D (Intelligent-Tiering) affects storage cost optimization, not ingestion throttling.