Which statement accurately describes Cisco ACI flooding mode for unknown unicast traffic?

This describes traditional Ethernet bridging behavior, but ACI's default behavior for unknown unicast is proxy forwarding, not flooding the entire bridge domain.

ACI does not drop all unknown unicast traffic by default; instead, it attempts to resolve the destination via the spine proxy.

While a hardware proxy is used, the statement "A hardware proxy must be used for flooding mode" is inaccurate because the proxy is used to avoid flooding, not to perform flooding itself.

By default, Cisco ACI uses a hardware proxy on the spine switches to handle unknown unicast traffic. When a leaf switch receives unknown unicast traffic, it encapsulates it and sends it to the spine proxy, which then performs an endpoint lookup to determine the correct destination leaf and forward the traffic.