Which root cause of the issue is true?

Refer to the exhibit.

A remote office was recently connected to the main office by using a GRE tunnel. Path MTU Discovery (PMTUD) is enabled on the tunnel interface. End users at the remote office report having issues accessing a file sever in the main office. Attempts to use PMTUD are failing.

Which root cause of the issue is true?
A . The MTU of the network path is 1500 bytes
B . ICMP redirect messages are enabled on a router interface in the network path
C . The MTU for Tunnel0 is set to 1400 bytes
D . The no ip unreachable command was applied to a router interface in the network path

Answer: D