In the cyber kill chain model, establishing a backdoor for persistent access falls under the 'Command and Control (C2)' stage. During this stage, attackers establish a method to continuously govern the compromised system or network, often by creating hidden access paths that ensure their continued control. 'Reconnaissance' is incorrect because it is the first stage where attackers gather information before launching an attack. 'Delivery' is also incorrect; it refers to the stage where the attack vector, like a phishing email or a malware-infested file, is delivered to the target. 'Weaponization' is the stage where an attacker combines an exploit with a backdoor into a deliverable payload, which precedes delivery and exploitation.