Which of the following best describes the ICMP "fragmentation required, DF flag set" message? $($DF is short for don't fragment.$)$

Group of answer choices

A host timed out in the reassembly of fragments.

A router has the DF flag set and cannot fragment.

A router discovered a larger MTU and the datagram must be fragmented, but the DF flag is set.

A router discovered a smaller MTU and the datagram must be fragmented, but the DF flag is set.