The application experiences unpredictable traffic spikes, which exceed the provisioned read and write capacity of the DynamoDB table, resulting in throttling errors. This is a classic scenario where on-demand capacity mode is the most appropriate solution.

Option A is correct because DynamoDB on-demand capacity automatically scales to handle sudden increases in traffic without requiring capacity planning. This eliminates throttling during unpredictable spikes and removes the operational burden of managing RCUs and WCUs. On-demand mode is particularly well-suited for workloads with variable or spiky access patterns.

Option B is incorrect because DynamoDB does not support read replicas in the same way as relational databases. Option C reduces cost for predictable workloads but does not prevent throttling if capacity limits are exceeded. Option D increases read cost and does not address capacity constraints.

Therefore, A is the best solution to improve resilience, eliminate throttling errors, and ensure a smooth user experience during traffic surges.