Why should iBGP sessions be fully meshed within a Transit AS?

A.
BGP requires redundant TCP sessions between iBGP peers.

B.
A full mesh allows for optimal routing within the Transit AS.

C.
Routes learned via iBGP are never propagated to other eBGP peers.

D.
Routes learned via iBGP are never propagated to other iBGP peers.

E.
Routes learned via eBGP are never propagated to other iBGP peers.

Explanation:
BGP split-horizon rule states that a route learned from one IBGP neighbor will not be
advertised to another IBGP neighbor so IBGP sessions be fully meshed. For example in
the topology below, routes learned from R1 about AS200 will be advertised to R2. For R2,
the route learned from R1 is a route learned from one IBGP neighborso R2 will not
advertise this route to R3 and AS 100 can never be a transit AS -> IBGP sessions should be
fully meshed within a transit AS.