What can cause a route learned through IBGP to be marked as hidden?

A.
unreachable next hop

B.
local preference

C.
AS path loop

D.
route preference

Explanation:
Hidden Paths due to i-BGP Hierarchy

Depending on the i-BGP architecture and the internal router topology, the number of paths learned by a router to reach a destination can differ. Because only the best paths are further propagated from one side of sub-AS (or route reflector) boundary to the other side, the number of overall paths learned can be further reduced.

Although all equally preferred path are announced into the route reflector by the border routers, the route reflector chooses only one best path based on its topology-dependent BGP best path selection criteria and propagates only the selected path to its clients, preventing the clients from learning all BGP-level equally preferred paths.