When a host on network A sends a message to a host on network B, which address does the router look at?
Answer :IP Address
The MAC or Ethernet address is associated with Layer 2 which is Data Link Layer of the OSI Model. The logical address is a 32-bit IP address (on iPv4 and 128 bits on iPv6) that is not embedded in the network card but it is assigned to it for the purpose of routing between networks.
Router operates at third layer of OSI model
It is used to connect two different networks so its operation is based on IP address.
Additional info:It can not transfer data between different model so it is not multi protocol converter