The described pop-up exhibits classic signs of scareware or a rogue security program. These malicious applications generate fake warnings to trick users into installing more malware or paying for useless software. Clicking "OK" would likely trigger the malware installation. The correct and safe first step is to investigate the alert's validity using the system's legitimate security tools. By checking the actual status of Windows Security (or another installed antivirus), the user can verify if a real threat exists without interacting with the potentially malicious pop-up.

A. Clicking the "OK" button is dangerous as it would likely approve the installation of malware disguised as an antivirus update.

B. Reinstalling the operating system is a drastic, last-resort measure for malware removal and is not the appropriate next step.

C. System File Checker (SFC) repairs corrupted Windows system files, but it is not a tool designed to detect or remove malware.

1. Microsoft Support. (2023). Protect yourself from tech support scams. Microsoft. In the section "How tech support scams work," it describes tactics like "scareware" pop-ups that urge users to take action. The recommended action is to not click and to use legitimate security software to scan the device.

2. Federal Trade Commission (FTC). (2022, May). How To Spot, Avoid, and Report Tech Support Scams. In the section "What a tech support scam looks like," the FTC warns about unexpected security alerts and advises users to check with their actual security software.

3. Carnegie Mellon University, Software Engineering Institute. (2018). Socially-Engineered Scams. CERT Division. This resource details how attackers use social engineering, including fake security alerts, to manipulate users into compromising their systems.