Which configuration ensures that the branch router in AS 45123 advertises only the local networks to all SP neighbors?

After some changes in the routing policy, it is noticed that the router in AS 45123 is being used as a transit AS router for several service provides .

Which configuration ensures that the branch router in AS 45123 advertises only the local networks to all SP neighbors?
A . ip as-path access-list 1 permit ^45123$
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out
B . ip as-path access-list 1 permit ^45123
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out
C . ip as-path access-list 1 permit ^$
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out
D . ip as-path access-list 1 permit .*
!
router bgp 45123
neighbor SP-Neighbors filter-list 1 out

View Answer

Answer: C

Explanation:

By default BGP advertises all prefixes to external BGP neighbors. This means that if you are multi-homed (connected to two or more ISPs) then you might become a transit AS. For example, ISP 2 in AS 200 can send traffic to your router in AS 100 to reach ISP 3 in AS 300 because you advertised prefixes in ISP 3 to ISP 2.

This is what will be seen in the BGP routing table of ISP1: