In BGP routing, what does the rule of synchronization mean?

A.
A BGP router can only advertise an EBGP learned route, provided that the route is an IGP
route in the routing table.

B.
A BGP router can only advertise an IBGP learned route, provided that the route is an IGP route
in the routing table.

C.
A BGP router can only advertise an IBGP learned route, provided that the route is an IGP route
that is not in the routing table.

D.
A BGP router can only advertise an EBGP learned route, provided that the route is a metric of 0
in the BGP table.

Explanation:
When an AS provides transit service to other ASs and if there are non-BGP routers in the AS,
transit traffic might be dropped if the intermediate non-BGP routers have not learned routes for
that traffic via an IGP. The BGP synchronization rule states that if an AS provides transit service to
another AS, BGP should not advertise a route until all of the routers within the AS have learned
about the route via an IGP. The topology shown in demonstrates the synchronization rule