The correct answer is 802.11ac, so option C is correct. In standard WLAN evolution knowledge used in HCIA-Datacom, 802.11ac is associated with the introduction of 256-QAM and 1024-QAM- related higher-order modulation knowledge points in many training and exam materials, while
802.11ax and 802.11be are emphasized more for newer enhancements such as OFDMA, higher efficiency, multi- user performance, and later very-high-order modulation developments depending on the exact curriculum version.
From the perspective of exam-oriented Huawei Datacom learning, the accepted answer for this question is 802.11ac.