R1 and R2 are configured as eBGP neighbor , R1 is in AS100 and R2 is in AS200. R2 is advertising these networks to R1:

The network administrator on R1 must improve convergence by blocking all subnets of 172-16.0.0/16 major network with a mask lower than 23 from coming in, Which set of configurations accomplishes the task on R1?
Correct Answer: A
"Blocking all subnets of 172.16.0.0/16 major network with a mask lower than 23 from coming in" would block 172.16.16.0/20.
The first prefix-list "ip prefix-list PL-1 deny 172.16.0.0/16 le 23" means "all networks that fall within the 172.16.0.0/16 range AND that have a subnet mask of /23 or less" are denied.
The second prefix-list "ip prefix-list PL-1 permit 0.0.0.0/0 le 32" means allows all other prefixes.