Refer to the exhibit.

What is the potential issue with this configuration?

A.
There is no potential issue; OSPF will work fine in any condition.

B.
Sub-optimal routing may occur since there is no area 1 adjacency between the ABRs.

C.
This is a wrong OSPF configuration because all routers must be in area 0 only.

D.
This is a wrong OSPF configuration because /30 requires 0.0.0.3 wild card.

Explanation:
The primary difference is that the GRE tunnel hides the real recipients under the outer IP header.
The “transit” area, in that case, does not need to actually know all the prefixes. What it needs,
though, is a default route that points towards the backbone. Suboptimal routing may occur, then,
because the traffic from the “transit” area must first reach the backbone in order to get GREencapsulated and carried again through the “transit” area to the disconnected area.