Refer to the exhibit. Which address range efficiently summarizes the routing table of the addresses for Router Main?

A /21 summary only spans 172.16.0.0 to 172.16.7.255 (8 networks), which is too small to cover all routes in the routing table and would leave some networks unsummarized.

A /20 prefix (172.16.0.0/20) covers the range 172.16.0.0 to 172.16.15.255, which encompasses exactly the 16 contiguous /24 networks present in Router Main's routing table. This is the most specific single summary that includes all routes without advertising unused address space beyond the actual table entries.

A /16 summary covers the entire 172.16.0.0 to 172.16.255.255 range (256 class-C networks), which is far broader than needed and would advertise a large block of non-existent subnets.

A /18 summary spans 172.16.0.0 to 172.16.63.255 (64 networks), which greatly exceeds the actual route count and advertises many subnets not present in the routing table.