The correct answers are C, D, and E .

The study guide lists the exact OSPF adjacency requirements :

“Interfaces of peers are the same type and in the same OSPF area”

“Each peer has a unique router ID”

“OSPF authentication, if enabled, is successful”

These map directly to:

C. OSPF interface network types match

D. Authentication settings match

E. OSPF router IDs are unique

The same study-guide slide also states other requirements such as:

peers’ primary IPs must be in the same subnet with the same mask

hello and dead intervals must match

OSPF MTUs must match

Why the other options are wrong:

A is wrong because link cost matching is not listed as an adjacency requirement . OSPF cost affects path selection, not whether adjacency can form.

B is wrong because interface priority does not need to be unique . Priority is used for DR/BDR election, where the highest priority wins, and ties are broken by router ID.

So the verified answers are: C, D, E .