Explanation
The use of access control lists (ACLs) is the most effective method to mitigate security risk for routers because they can limit Telnet and traffic from the open Internet. Telnet is a protocol that allows remote access to a device, which can pose a security threat if not properly controlled. Traffic from the open Internet can also contain malicious packets that can harm the network or the router itself. ACLs act as filters that can block or allow specific types of traffic based on predefined criteria, such as source and destination addresses, protocols, ports, and flags. By using ACLs, routers can prevent unauthorized access and reduce the exposure to potential attacks.
References:
Protecting Your Core: Infrastructure Protection Access Control Lists
Definition, purposes, benefits, and functions of ACL
CISA Review Manual 27th Edition, page 336