Answer: (ii)

ICMP Redirect (type 5) is sent by a router to the original source host when it forwards a datagram and finds that the next hop it used is not the best router for that destination - in other words, the packet appears to have been routed wrongly and the host should use a different router for subsequent packets to that destination. Statement (ii) describes exactly this.

The other statements are not valid: (i) is false because ICMP does not report every transmission error - it reports datagram-handling problems (destination unreachable, time exceeded, parameter problem, etc.), and by design it never reports an error contained inside an ICMP error message itself. (iii) misstates the recipient: redirect messages go to the sending host, not to routers, and routers must not alter their routing tables from redirects. (iv) misstates the trigger: Destination Unreachable (type 3) means the datagram could not be delivered - network unreachable, host unreachable, protocol unreachable, port unreachable, fragmentation needed - and it is generated by end hosts as well as routers, so it is not simply the case of a router being unable to locate the destination.