The correct solution is C. AWS IoT Core is a fully managed service specifically designed to enable secure, bi-directional communication between internet-connected devices and the AWS Cloud. It supports the MQTT protocol and includes built-in device authentication and access control, which significantly minimizes operational overhead. By setting up AWS IoT Core, creating an AWS IoT thing for each device, and provisioning a certificate, the company can seamlessly manage thousands of devices with unique X.509 certificates. This approach is more efficient and less complex compared to other options, which involve additional services and increased operational management.