Which of the following three options are supported APIs of IOS XR?

The correct answer, indicating that NETCONF is a supported API of IOS XR, is based on its design as a network management protocol specifically tailored for device configuration and management. NETCONF utilizes XML to convey data and supports mechanisms for querying and modifying device configurations. This protocol allows for robust network automation and programmability in IOS XR environments.

While other options may appear relevant to network programmability, they do not stand out as core APIs explicitly designated for IOS XR within the context of industry standards. RESTCONF, for instance, is designed for RESTful access to network devices using YANG models, and although it may offer some flexibility, it isn't categorized as a primary API for IOS XR compared to NETCONF. Similarly, gRPC is a modern RPC framework which provides efficient communication protocols but is generally not designated specifically for IOS XR management in the same way that NETCONF is. XR-API, while sounding relevant, lacks prominent recognition or standardization compared to NETCONF, RESTCONF, and gRPC, making it less applicable in this context.

Therefore, the identification of NETCONF is not only correct but aligns well with the underlying architecture and operational capabilities of IOS XR, confirming its role as a foundational component for network management.