To ensure voice and data traffic are properly prioritized for an IP phone, the administrator should modify (configure) QoS. In Network+ (N10-009), QoS is the primary mechanism used to prioritize latency-sensitive traffic such as VoIP. By classifying and marking voice frames/packets (often using DSCP/CoS values) and applying priority queuing, the network ensures voice traffic experiences minimal delay and jitter even when links are congested. This is essential for call quality because voice is highly sensitive to variation in delivery time and packet loss.
802.1Q tagging is important for VLAN separation (and many IP phone deployments use a voice VLAN), but VLAN tagging alone does not guarantee prioritization; it separates traffic, while QoS provides preferential treatment. Full duplex can reduce collisions on Ethernet links, but it does not implement traffic prioritization policies. Changing the native VLAN is a trunking/security configuration detail and not the correct action to prioritize voice versus data. In practice, networks often use both a voice VLAN (802.1Q) and QoS, but for the explicit requirement of prioritization, QoS is the correct answer.