When there's an existing network management system (NMS) already doing network monitoring, the appropriate ISE probe is SNMP - specifically the SNMP Query probe, which allows ISE to query network devices (switches, WLCs) to gather endpoint information such as MAC addresses, connected ports, and VLAN data.
SNMP integrates naturally with an existing NMS infrastructure during the discovery phase. The other options are distractors: NMAP is active scanning, RADIUS requires authentication events to already be flowing, and NetFlow is for traffic analysis rather than endpoint discovery.