Final answer:
OnePK is a Cisco proprietary API that enables developers to access Cisco network devices' features, making it the correct answer.
Step-by-step explanation:
The southbound API that matches the description of being a Cisco proprietary API is OnePK (Open Network Environment Platform Kit). While NETCONF is a network configuration protocol often used for router/switch configurations and OpenFlow is a protocol providing access to the forwarding plane of a network switch or router, OpFlex is a Cisco initiative for policy enforcement. However, OnePK was specifically designed by Cisco to enable developers to access the features and functionalities of Cisco network devices, making it the correct answer to the question.