Comprehensive and Detailed 250 to 300 words of Explanation (AWS documentation-based, no links):

The key constraint is no application code changes, while the observed failure mode during spikes is heavy write load on the database that makes the site unresponsive. Scaling the web tier alone (Option B) may not help if the database is the bottleneck; in fact, adding more EC2 instances can increase concurrent write pressure and worsen the database overload. Caching (Option A) would require modifying the application to use Redis for query caching, which violates the no-code-change requirement. Option C is not a fit: CloudFront caching is effective for cacheable content, but authenticated, personalized pages are typically not cacheable without application changes, and “write throttle” is not a standard CloudFront feature to protect the database.

Migrating from RDS for MySQL to Amazon Aurora Serverless is a managed approach designed to automatically adjust database capacity to match workload demand, which is well suited to “low baseline traffic with occasional sudden spikes.” By setting an appropriate maximum capacity, Aurora Serverless can scale to handle bursts more gracefully than a fixed-size RDS instance, improving availability during unpredictable demand. This reduces operational overhead because capacity management is largely handled by the service rather than by manual instance resizing or complex scaling scripts.

The final step—configuring EC2 instances to connect to the new Aurora endpoint—is an infrastructure configuration change, not an application code rewrite. The application continues to speak the same MySQL-compatible protocol (Aurora MySQL-compatible), preserving compatibility while improving resilience to spikes.

Therefore, D best meets the requirement to improve availability during sudden traffic increases driven by heavy database writes, with minimal operational effort and no application code changes.