Choice "A" is correct. Circuit level gateways, not packet filtering, only allow data into a network that result from requests from computers inside the network by keeping track of requests that are sent out of the network and only allowing data in that is in response to those requests. Choice "c" is incorrect. Packet filtering examines packets of data as they pass through the firewall. Packet filtering is the simplest, not the most complex, type of firewall configuration.Choice "b" is incorrect. Application level gateways examine data coming into the gateway. They can be used to control which computers in a network can access the Internet and can be used to control which Internet websites or pages can be viewed once access is allowed.Choice "d" is incorrect. Choice "A" is the best answer.