What is a stateful firewall?

One common requirement for personal firewalls is that they are stateful. What is the most significant characteristic of a stateful firewall?

A) The firewall keeps track of the state of each traffic flow and only allows traffic that matches an existing connection state

B) The firewall relies on static rules to determine the state of incoming traffic

C) The firewall identifies potential attackers and their attack vectors

D) The firewall understands and provides access based on the state of the remote device