The question presents a scenario where a solutions architect is tasked with designing a data storage and retrieval architecture for a new application. The application is expected to handle a high volume of data, with millions of small records (each less than 4 KB) being ingested per minute from devices worldwide. The data is described as ephemeral, with a requirement to store it for a duration of 120 days before it can be discarded.

The correct answer to the question, as indicated in the file content, is option C. This solution involves setting up AWS IoT Core, creating a corresponding AWS IoT thing for each device, and provisioning a certificate for each device. The devices would then be connected to AWS IoT Core.

Here's a comprehensive explanation of why option C is the most suitable solution:

1. Scalability: AWS IoT Core is designed to handle large numbers of devices and messages, making it scalable for applications that need to ingest millions of records per minute.

2. Security: By creating a corresponding AWS IoT thing and provisioning a certificate for each device, the solution ensures secure communication between the devices and the AWS IoT Core.

3. Durability: AWS IoT Core provides a durable storage mechanism for the data, which is essential for the application's requirement to store data for 120 days.

4. Latency: AWS IoT Core is optimized for low-latency data retrieval, which aligns with the application's need for quick access to the stored records.

5. Operational Overhead: Among the given options, setting up AWS IoT Core with AWS IoT things and certificates likely involves the least operational overhead compared to managing an MQTT broker on EC2 instances or setting up an API Gateway with a Network Load Balancer.

6. Cost-Effectiveness: AWS IoT Core is a managed service, which means it can be more cost-effective than running and maintaining an MQTT broker or an API Gateway setup, especially at the scale required by the application.

In summary, option C meets the requirements of the application with the least operational overhead by leveraging AWS IoT Core's scalability, security, durability, and low-latency data retrieval capabilities, while also being cost-effective.