How does a BGP router process the received route whose next hop is unreachable? 

The BGP router sends an error message to the router that has sent the Update message. 

The BGP router accepts this route and adds it to the BGP routing table. This route, however, is neither preferred nor added to the IP routing table for packet forwarding. 

The BGP router refuses this route. 

The BGP router accepts this route and adds it to the BGP routing table. This route can participate in route selection and may be selected as the optimal route to be added to the IP routing table for packet forwarding.