A.All of the routers need to be configured for backbone Area 1.
B.R1 and R2 are the DR and BDR, so OSPF will not establish neighbor adjacency with R3.
C.A static route has been configured from R1 to R3 and prevents the neighbor adjacency from being established.
D.The hello and dead interval timers are not set to the same values on R1 and R3.
E.EIGRP is also configured on these routers with a lower administrative distance.
F.R1 and R3 are configured in different areas.