Answer: (iii) TTL, Options, Datagram Length, Offset

The IP header checksum is computed over the IP header only (the payload is excluded), so any change to a header field invalidates the stored checksum. A router always decrements the TTL field before forwarding, which by itself forces recomputation. The Options fields (record route, timestamp, etc.) may be updated by routers along the path. If the router must fragment the datagram, every fragment gets a new datagram (total) length and a new fragment offset, so each new header needs a fresh checksum.

The data (ii) is not covered by the header checksum, so it can never trigger recomputation. The identification number (i) is copied unchanged from the parent datagram into each fragment - its job is to tie the fragments together - so it does not change during forwarding. Header length and ToS (iv) are also not rewritten by a normal forwarding router, so they are not the reason for per-hop recomputation.