Which two methods change the IP MTU value for an interface?

An IOS device configured for IP+MPLS routing uses three different Maximum Transmission Unit
(MTU) values: The hardware MTU configured with the mtu interface configuration command
The hardware MTU specifies the maximum packet length the interface can support … or at least
that’s the theory behind it. In reality, longer packets can be sent (assuming the hardware interface
chipset doesn’t complain); therefore you can configure MPLS MTU to be larger than the interface MTU and still have a working network. Oversized packets might not be received correctly if the
interface uses fixed-length buffers; platforms with scatter/gather architecture (also called particle
buffers) usually survive incoming oversized packets.
IP MTU is used to determine whether am IP packet forwarded through an interface has to be
fragmented. It has to be lower or equal to hardware MTU (and this limitation is enforced). If it
equals the HW MTU, its value does not appear in the running configuration and it tracks the
changes in HW MTU. For example, if you configure ip mtu 1300 on a Serial interface, it will appear
in the running configuration as long as the hardware MTU is not equal to 1300 (and will not
change as the HW MTU changes). However, as soon as the mtu 1300 is configured, the ip mtu
1300 command disappears from the configuration and the IP MTU yet again tracks the HW MTU.
Reference. http://blog.ipspace.net/2007/10/tale-of-three-mtus.html

C: refer to hardware MTU, limited by hardware
D: refer to the configured MTU, when it is equal to hardware MTU (default), it will not appear in the config