Which API must an engineer use to change a netmask on a Cisco IOS XE...

RESTCONF and YANG are used for network configuration and management on Cisco IOS XE devices. RESTCONF provides a REST-like interface for accessing YANG-defined data models, allowing engineers to change configurations such as netmasks through programmable interfaces.

References:

Cisco DevNet Associate Study Guide: Network Programmability and Automation (Chapter 6, Section: Using RESTCONF and YANG).