A TFTP download from the CPE's flash over Gi0/0 fails while an inbound ACL is applied. Which action resolves the transfer failure?

The correct answer is to change the WAN ACL to permit the entire UDP destination port range because TFTP uses a well-known source port (69) but a dynamically assigned, ephemeral destination port for data transfer. An inbound ACL on the Gi0/0 interface would block these dynamic destination ports if only port 69 is permitted. Permitting the entire UDP destination port range (or at least a wide range of high-numbered ports) allows the TFTP data transfer to complete. Making the permit udp any eq tftp any entry the last entry in the WAN ACL would not resolve the issue, as the order does not change the fact that the dynamic destination ports are still blocked. Shortening the file name is irrelevant to network connectivity issues caused by ACLs. Changing the WAN ACL to permit only UDP port 69 would still block the dynamic destination ports used for the data transfer, as port 69 is typically only for the initial connection request.