A pair of routers are in EIGRP named mode. One router is running 'router eigrp CORP' and has not entered address-family ipv4 unicast configuration, the other has. Both are otherwise correctly reachable. What is the most likely reason IPv4 EIGRP adjacencies do not form between them?

In EIGRP named mode, simply defining router eigrp CORP is not enough to enable IPv4 routing. You must explicitly enter the address-family ipv4 unicast configuration mode within the named EIGRP instance to activate IPv4 routing and allow adjacencies to form. Without this, the router will not process or send IPv4 EIGRP updates, preventing adjacency with a router that has configured the address family. The other options are incorrect: EIGRP named mode is designed to replace classic mode, not require it for IPv4 adjacencies. Named mode does not inherently require manual redistribution between instances for basic adjacency formation. There is no global command eigrp ipv4 enable required for IPv4 EIGRP adjacencies in named mode; the address-family configuration handles this.