The issue described indicates that even though 172.20.20.2 stops responding to ICMP echoes, the default route does not get removed as expected. This suggests a problem with the tracking configuration of the route. The correct implementation should include associating the tracked object with the routing configuration so that if the tracked object goes down, it triggers the removal of the associated route. Since option C points out that the default route lacks this association with tracking, it is identified as the cause of the issue.