When Task Manager repeatedly fails to open, that's a strong red flag for system tampering-especially malware designed to block diagnostic and remediation tools. Quentin Docter explains that stealth malware attempts to avoid detection by "masking itself from applications" and can redirect commands around itself to avoid detection. That behavior fits the symptom of a core troubleshooting utility (Task Manager) not launching when an administrator needs it.
Docter also reinforces that the correct process in suspected malware cases begins with "identify and verify the malware symptoms," followed by containment and remediation steps. If a key system tool is blocked, that itself is a "verify symptoms" indicator that the system is compromised and needs malware-handling procedures.
The other choices don't explain the symptom well: a dev app running would not normally prevent Task Manager from launching; Windows Update set to manual wouldn't block Task Manager; and a moved pagefile might affect performance but not typically stop Task Manager from opening at all. Therefore, malware infection is the most likely cause.