The correct answer is orthogonal frequency-division multiple access, or OFDMA. Cisco identifies 802.11ax, also known as Wi-Fi 6, as the generation that introduces OFDMA-based operation to improve efficiency, especially in dense wireless environments. Cisco's Catalyst 9800 documentation states that OFDMA support for 802.11ax APs improves performance in dense deployments, enables simultaneous data transmissions with OFDMA and MU-MIMO, and supports uplink and downlink multi-user operations across multiple client devices.

Strictly, OFDMA is a multi-user channel-access and subcarrier allocation method rather than a pure constellation modulation scheme. However, within 802.11ax fundamentals and this answer set, OFDMA is the defining PHY/MAC technology. It divides a 20, 40, 80, or 160 MHz channel into smaller resource units so multiple clients can transmit or receive in the same transmission opportunity. Cisco further explains that OFDMA improves wireless performance by using independently modulated subcarriers within frequencies, allowing simultaneous transmissions to and from multiple clients. FHSS and DSSS belong to older 802.11 PHY generations. QAM is used for symbol modulation, but ''Quadrature Amplitude Modulation OFDMA'' is not the primary 802.11ax technology name. Reference topics: 802.11ax/Wi-Fi 6, OFDMA, resource units, uplink/downlink multi-user operation, and high-efficiency WLAN design.