An operator configures a LAN-facing interface as passive for EIGRP. Which immediate effect will this configuration have on that interface?

Configuring an interface as passive in EIGRP prevents it from sending or receiving EIGRP Hello packets. This means the router will not attempt to form EIGRP adjacencies with other routers on that specific interface. However, the network directly connected to that passive interface will still be included in the EIGRP routing updates sent out through other active EIGRP interfaces. This is useful for interfaces connected to end-user segments where EIGRP adjacencies are not needed, saving bandwidth and CPU cycles. The other options are incorrect: EIGRP cannot form neighbors without Hellos, it does not shut down interfaces, and it does not convert connected routes to external routes simply by making an interface passive.