Amazon VPC IP Address Manager (IPAM) is a managed service specifically designed to plan, track, and monitor IP addresses for AWS workloads. By creating pools in IPAM and enabling auto-import, IPAM automatically discovers and tracks the IP address usage of existing VPCs and subnets. IPAM natively publishes utilization metrics to Amazon CloudWatch. Configuring a CloudWatch alarm on the IPAM pool's utilization metric is the most direct and efficient method to trigger an Amazon Simple Notification Service (Amazon SNS) notification when a specific threshold is breached. This approach provides a fully managed monitoring and alerting solution, fulfilling the requirements with the least operational overhead.

B. This solution requires developing, deploying, and maintaining a custom AWS Lambda function and associated logging/metric filtering, which creates significantly more operational overhead than using the purpose-built IPAM service.

C. Similar to option B, this approach relies on a custom Lambda function. While publishing custom metrics is more direct than logging, it still constitutes a custom-built solution with higher operational overhead than the managed IPAM service.

D. While IPAM is the correct service, Amazon EventBridge is primarily for event-driven architectures that react to state changes (e.g., a pool's state changes to 'depleted'). For continuous monitoring against a specific numeric threshold, Amazon CloudWatch alarms are the more appropriate and direct tool.

1. AWS Documentation, VPC IP Address Manager (IPAM) User Guide, "Monitor IP address usage with IPAM": "VPC IPAM makes it easier for you to monitor IP address usage throughout your network and provides alerts when there is a potential for IP address exhaustion." This confirms IPAM is the purpose-built service for the scenario.

2. AWS Documentation, VPC IP Address Manager (IPAM) User Guide, "Monitor IPAM metrics using CloudWatch": This section details the metrics that IPAM sends to CloudWatch, including IpamPoolAllocation, which represents the percentage of the pool's address space that is allocated. It explicitly states, "You can use these metrics to set up alarms that notify you when you're close to exhausting the address space in a pool." This directly supports the solution in option A.

3. AWS Documentation, Amazon CloudWatch User Guide, "Using Amazon CloudWatch alarms": "You can create a CloudWatch alarm that watches a single CloudWatch metric... If the metric breaches the threshold for a specified number of evaluation periods, the alarm performs one or more actions," such as sending a notification to an SNS topic. This confirms the mechanism described in option A is the standard and intended method for threshold-based alerting.

4. AWS Well-Architected Framework, "Operational Excellence Pillar" (Whitepaper), Design Principle OPS 8: "Use managed services to reduce the operational burden of managing physical or virtual servers." This principle supports choosing a managed service like IPAM over a custom solution involving Lambda functions (as in options B and C) to minimize operational overhead.