Full duplex mode allows devices to send and receive data simultaneously, improving network performance and reducing congestion, which is critical for VoIP and video conferencing.

Breakdown of Options:

A. A VLAN with 100Mbps speed limits – VLANs segment traffic but limiting speeds to 100Mbps would worsen video performance.

B. An IP helper to direct VoIP traffic – IP helper is used for DHCP relay, not for VoIP optimization.

C. A smaller subnet mask – A smaller subnet reduces IP address availability but does not improve network performance.

D. Full duplex on all user ports – Correct answer. Full duplex eliminates collisions, allowing better VoIP and video performance.