Final Answer:
Option (a) "Defines tunnel search criteria" accurately describes the purpose of the "set tunnel-search selectors" command in the context of configuring IPsec VPNs.
Step-by-step explanation:
In the context of configuring VPNs using IPsec (Internet Protocol Security), the "set tunnel-search selectors" command is used to define tunnel search criteria. This command is part of the configuration for the phase1-interface settings in IPsec VPNs.
IPsec VPNs often involve the establishment of tunnels between different network devices. These tunnels need specific criteria to determine when they should be established or torn down. The "set tunnel-search selectors" command allows the definition of these criteria, specifying conditions that, when met, trigger the initiation or termination of a tunnel.