This is a compensating control because the organization cannot apply the preferred control, which would be patching or replacing the end-of-service server. Instead, it uses network segmentation to reduce exposure and limit the risk. A compensating control is an alternative safeguard used when the primary security requirement cannot be implemented due to technical, operational, or business constraints. Moving the server to a segmented network does not fix the vulnerability itself, so it is not corrective. It may reduce attack paths, but the key issue is that it substitutes for the missing patching capability. A deterrent control discourages unwanted behavior, such as warning signs or legal notices. A preventive control directly blocks incidents, but here the more precise classification is compensating.

Input validation (D)is the most effective way to preventinjection attacks, such asSQL injection, XSS, etc. It ensures that only correctly formatted and expected inputs are processed by the application.
This is clearly identified underDomain 2.3: Application security techniques, whereinput validationis listed as aprimary defense against injection attacks.
Reference: CompTIA Security+ SY0-701 Objectives, Domain 2.3 - "Input validation: Prevents injection and malformed data attacks."

Firewalls are classic examples of technical controls as they use technology to restrict or permit network traffic based on security policies. Technical controls are implemented through hardware or software solutions.
Security guards (A) and fences (C) are physical controls, while policies (B) are administrative controls.
Technical controls like firewalls are fundamental in securing network perimeters and endpoints as covered in the Security Architecture domain#6:Chapter 3 CompTIA Security+ Study Guide#.

A group policy object (GPO) is a mechanism for applying configuration settings to computers and users in an Active Directory domain. By pushing a GPO update, the systems administrator can quickly and uniformly enforce the new password policy across all systems in the domain. Deploying PowerShell scripts, enabling PAP, and updating EDR profiles are not the most efficient or effective ways to change the password policy within an enterprise environment. References: CompTIA Security+ Study Guide: Exam SY0-701, 9th Edition, page 115; Password Policy - Windows Security