Which two utilities can be used to interface with the NETCONF API on IOS XR?

The NETCONF API is designed to enable configuration management and automation of network devices through a standardized method, and there are specific utilities that facilitate this interaction on IOS XR. The choice of 'netconf server (sub system)' is correct because it provides the underlying framework for supporting NETCONF operations on Cisco IOS XR devices.

The NETCONF server capability ensures that the device can respond to NETCONF operations like session initiation, configuration retrieval, and management of configuration data. This subsystem on IOS XR allows users to programmatically manage the device, making it indispensable for automating network configurations and operational tasks.

Other utilities may help interact with NETCONF, but they rely on the existence of a NETCONF server to function correctly. For instance, applications like ncclient are client libraries that facilitate the interaction with a NETCONF server from a programming aspect, typically used in Python environments. However, they cannot function independently as they require the server component to communicate with network devices.

In summary, the netconf server subsystem is crucial for enabling NETCONF on IOS XR, thus making it the right choice in this context for interfacing with the protocol.