The selection of countermeasures is driven by the output from a risk assessment, which identifies the risks and their associated likelihood and consequences for each zone and conduit in the industrial automation and control system (IACS). The risk assessment also determines the target security level (SL-T) for each zone and conduit, which represents the desired level of protection against the identified threats. The countermeasures are then selected based on the SL-T and the existing security level (SL-A) of the zone and conduit, as well as the cost and feasibility of implementation. The countermeasures should aim to reduce the risk to an acceptable level by increasing the SL-A to meet or exceed the SL-T. References: ISA/IEC 62443-3-2:2018 - Security risk assessment for system design, ISA/IEC 62443-3-3:2013 - System security requirements and security levels, ISA/IEC 62443 Cybersecurity Fundamentals Specialist Training Course

According to ISA/IEC 62443-1-1, a vulnerability is defined as "the potential for violation of security," which means a weakness or gap in protection efforts that could be exploited by threats to gain unauthorized access or cause harm to an IACS. It does not specifically mean an event (B) or a result (D), and it is broader than just management flaws (A). The identification and management of vulnerabilities are key steps in risk assessment and mitigation in the 62443 framework.
Reference: ISA/IEC 62443-1-1:2007, Section 3.3, Glossary ("vulnerability" definition).

In cybersecurity, a demilitarized zone (DMZ) refers to a physical or logical subnetwork that contains and exposes an organization's external-facing services to an untrusted network, typically the internet. The main characteristic of a DMZ is that it acts as a buffer zone between the public internet and the private network.
This allows for internet access through the firewall while keeping the internal network secure. Internet-facing servers are placed in the DMZ so that they are separated from the rest of the internal network. By doing so, if a server in the DMZ is compromised, the attacker would not have direct access to the internal network. This architecture is commonly used to host services such as web servers, mail servers, and FTP servers. Choice C is the most closely associated with the deployment of a DMZ as it allows for regulated and monitored internet access through a firewall.

SP Element 6 in ISA/IEC 62443-2-1 covers User Access Control. Within this, USER 1 - Identification and Authentication includes controls such as:
Unique user identification
Password and credential management
Authentication mechanisms
"USER 1 defines policies for individual user identification and password protection to enforce accountability and prevent unauthorized access."
- ISA/IEC 62443-2-1:2010, Clause 4.3.4 - SP Element 6
"Password protection" is a core component of this control, while other options (like incident handling or backup) fall under different SP elements.
References:
ISA/IEC 62443-2-1:2010 - SP Element 6, USER 1
ISA/IEC 62443-1-1 - Definitions of authentication and access control

The Triton (also known as Trisis) malware specifically targeted and compromised Safety Instrumented Systems (SIS), disabling the emergency shutdown capabilities at a petrochemical plant. This attack demonstrated that safety systems, once considered isolated and secure, are vulnerable to sophisticated, targeted cyberattacks. Neither Zeus, Stuxnet, nor WannaCry had this specific impact on emergency shutdown safety systems.
Reference: ISA/IEC 62443-1-1:2007, Section 3.2.4; ISA/IEC 62443-3-3:2013, Section 4.2.2; Public incident reports and analyses of the Triton/Trisis attack (e.g., FireEye 2017 case study).