What is the reason firewalls are considered stateful?

Firewalls keep track of the TCP conversation before and after the three-way handshake. This is done so that an attack on the TCP/UDP flow is not executed; in addition, denial-of-service (DoS) attacks can be thwarted, such as a SYN flood. Zone state is terminology that is used with firewalls; therefore, it is an incorrect answer. Firewalls do not protect by keeping statistics or accounting information for the state of packets. Firewalls do not transition between defense states.