I. Datagrams at the source must be the size of the smallest maximum transmission unit (MTU) of all the links on a path to the destination. $\rightarrow$ False.
This process is called Path MTU Detection Algorithm(PMTUD Algorithm) which is not available in IPv4. But PMTUD algorithm is run by every source host in IPv6 before sending data, to avoid the overhead of fragmentation at the intermediate routers.
II. Datagrams may be fragmented during routing. $\rightarrow$ True.
III. Datagrams are reassembled only at the destination. $\rightarrow$ True.