Which feature ensures seamless roaming between APs with no noticeable delay for voice clients?

Client load balancing distributes new associations across APs for capacity management and has no effect on roaming latency.

802.11k provides Radio Resource Management neighbor reports that help a client identify candidate APs, but it does not accelerate the authentication process when transitioning.

802.11r pre-caches the Pairwise Master Key Security Association (PMKSA) at neighboring APs before the client roams, reducing the full 802.1X re-authentication to a Fast BSS Transition exchange. This cuts roaming latency to under 50 ms, which is within the threshold required for uninterrupted VoIP - typically less than 150 ms end-to-end delay.

802.11d is the regulatory domain amendment that advertises country-specific channel and power constraints; it has no roaming or authentication function.