A site redistributes OSPF routes into EIGRP and tags them with 100. To prevent those tagged EIGRP routes from being redistributed back into OSPF, which configuration is appropriate?

The correct approach is to use a route-map during redistribution from EIGRP back into OSPF. This route-map should deny routes that have been tagged with 100. This prevents a routing loop where routes originating in OSPF, redistributed into EIGRP, and then tagged, are not re-injected into OSPF. A distribute-list under OSPF filters incoming or outgoing OSPF routes, but it doesn't prevent redistribution from another protocol. Lowering OSPF administrative distance only affects route preference, not redistribution behavior. IP community lists are used with BGP, not EIGRP or OSPF tags.