The correct answer is D. Firewall.
UNIX, MS-DOS, and Windows are all operating systems. An operating system is a software program that manages a computer’s hardware and software resources and provides a platform for running other programs. A firewall is a security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules.
UNIX is a multitasking, multiuser computer operating system that exists in many variants. The original Unix was developed at AT&T’s Bell Labs research center by Ken Thompson, Dennis Ritchie, and others. Unix was first released in 1971, and it has been widely adopted in the scientific, engineering, and business communities.
MS-DOS is a disk operating system that was developed by Microsoft for IBM personal computers. MS-DOS was first released in 1981, and it was the most popular operating system for personal computers until the release of Windows in 1985.
Windows is a family of operating systems developed by Microsoft. The first version of Windows was released in 1985, and it has been through many iterations since then. Windows is the most popular operating system for personal computers in the world.
A firewall is a security system that monitors and controls incoming and outgoing network traffic based on predetermined security rules. Firewalls are used to protect computer networks from unauthorized access and malicious attacks.