Final answer:
The most effective way to prevent others from viewing a user's computer screen is to use a privacy filter, which limits the screen's viewing angle.
Step-by-step explanation:
The best option to prevent other people from viewing a user's computer screen is D. Privacy filter. Privacy filters are physical layers that you place over a computer screen to limit the viewing angle, so only the person sitting directly in front of the monitor can see the content clearly. In contrast, anti-virus software protects against malware, a key fob is often used for secure access to systems or buildings, and biometric devices authenticate a person's identity based on physical characteristics. Therefore, only the privacy filter relates directly to screen visibility and privacy.