What is the default administrative distance for OSPF?

Correct answer: 110

The default administrative distance (AD) for OSPF in Cisco IOS is 110, which routers use to prefer one routing protocol's routes over another when multiple protocols learn the same destination. Option A is wrong because 90 is the default AD for EIGRP internal routes, not OSPF. Option C is wrong because 120 is the default AD for RIP (Routing Information Protocol), a distance-vector protocol. Option D is wrong because 170 is the default AD for EIGRP external routes, representing routes redistributed into EIGRP from other protocols.