The _____ is used by the local host to determine which hosts are on the local subnet, and which hosts are on remote networks.

A.
DNS

B.
ARP

C.
gateway

D.
netmask

E.
routing protocol

Explanation:
From http://en.wikipedia.org/wiki/Subnetwork
All computers belonging to a particular network must follow a simple rule for assigning addresses to their interfaces. This involves dividing the set of bits of an address into two parts:
1. The first part is the network prefix. It is a contiguous group of high-order bits whose value is common among all hosts within a network.
2. The remaining, least-significant bits of the address designate the host or interface identifier. This part is unique on the network and specifies the device or often just one specific network interface connected to the local network.
With this rule, IP packets may be selectively routed across multiple networks via routers to a destination host if the network prefixes of origination and destination hosts differ, or sent directly to a target host on the local network if they are the same.
The network prefix may be written in a form identical to that of the address itself. This is called the network mask, or netmask, of the address.