Computer $1$ on Network A sends a packet to Computer $2$ on Network C$.$ What's the last step that Router $Z$ does after receiving the Ethernet frame?

Strips away the Ethernet frame, leaving the IP datagram. Performs a checksum calculation against the entire datagram

Calculates a checksum and compares this checksum with the one in the Ethernet frame header

Decrements the TIL by $1,$ calculates a new checksum, and makes a newIP datagram. This new IP datagram is again encapsulated on a new Ethernet frame.

Sends back the packages to router $Y$ for confirmation