Configure reserved concurrency for Lambda function Beta to cap its maximum concurrent executions and prevent it from exhausting the account is correct. This setting limits Beta and preserves unreserved capacity so that Alpha can be invoked by the Application Load Balancer.

Reserved concurrency puts a hard ceiling on how many concurrent executions a single function can use and it also guarantees that the remaining account concurrency is available to other functions. By configuring reserved concurrency on Beta the team prevents Beta from consuming nearly all of the account pool and they allow Alpha to obtain the concurrency it needs when the ALB routes requests to it.

Enable provisioned concurrency on Lambda function Beta to restrict its concurrency during spikes is incorrect because provisioned concurrency allocates warm execution environments for predictable latency and it does not impose a concurrency limit. It can actually consume concurrency capacity and it does not act as a throttle.

Use Amazon API Gateway instead of an Application Load Balancer for Lambda function Alpha is incorrect because switching the frontend does not change the account level concurrency that Beta is exhausting. The concurrency contention is at the Lambda account level and a different trigger will not solve that.

Enable provisioned concurrency on Lambda function Alpha is incorrect because provisioned concurrency addresses cold starts and latency rather than preventing starvation when another function has consumed most of the account concurrency. It does not free capacity if the account pool is already exhausted.

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