Final answer:
To sniff unicast traffic from all hosts on a switch, one must configure port mirroring, which lets a designated port receive copies of network packets for monitoring purposes.
Step-by-step explanation:
To allow a host to sniff unicast traffic from all hosts connected to a switch on a switched network, the configuration change that must be made is B) Configure port mirroring on the switch. Port mirroring, also known as SPAN (Switched Port Analyzer), involves setting up a dedicated port that receives a copy of all network packets seen on one or more specified ports, allowing for monitoring or analysis. This configuration is particularly useful for network diagnostics and security monitoring. Enabling port security, increasing the VLAN membership of the host, or disabling the Spanning Tree Protocol (STP) would not accomplish this task, as they serve different network functions.