Host A (on TCP/IP v4 network A) sends an IP datagram D to host B (also on TCP/IP v4 network B). Assume that no error occurred during the transmission of D. When D reaches B, which of the following IP header field(s) may be different from that of the original datagram D? (i) TTL (ii) Checksum (iii) Fragment Offset

Let us examine each field when an IP datagram travels across routers:

1. TTL (Time to Live): Every router that forwards the datagram decrements the TTL value by at least $1$. Thus, TTL at Host B will be less than the initial TTL set by Host A.

2. Header Checksum: Since the TTL field changes at each router hop, the IPv4 header checksum must be recomputed and updated at every router.

3. Fragment Offset: If the datagram traverses a network link with a smaller MTU (Maximum Transmission Unit), intermediate routers may fragment the datagram, changing the Fragment Offset field in the fragments.

Therefore, all three fields ((i), (ii), and (iii)) may be different from the original datagram D. Thus, (D) is the correct option.