Assume that there is no option field in the header of an IPv4 datagram. Which field (s) of the IPv4 header change from router to router?

The fields of the IPv4 header change from router to router is:

No Fragmentation - TTL, checksum (due to TTL). Fragmentation - Total length field, fragment M bit.

Explanation:

TTL - TTL stands for Time-TO-Live, it is important in an IP (Internet Protocol) packets that says a network router if or not the packets have been in the network so lengthy and should be rejected.

Fragmentation - It is done through the network layer when the mostly size of data-gram is higher than max size of data that can held a frame.

It is done through the network layer at the end side and is generally done at routers.