To reduce the number of individual operating systems while decommissioning physical servers, the company should use containerization. Containerization allows multiple applications to run in isolated environments on a single operating system, significantly reducing the overhead compared to running multiple virtual machines, each with its own OS.
Containerization: Uses containers to run multiple isolated applications on a single OS kernel, reducing the need for multiple OS instances and improving resource utilization.
Microservices: An architectural style that structures an application as a collection of loosely coupled services, which does not necessarily reduce the number of operating systems.
Virtualization: Allows multiple virtual machines to run on a single physical server, but each VM requires its own OS, not reducing the number of OS instances.
Infrastructure as code: Manages and provisions computing infrastructure through machine-readable configuration files, but it does not directly impact the number of operating systems.

A rootkit is a type of malware that modifies or replaces system files or processes to hide its presence and activity. A rootkit can change the hash of the cmd.exe file, which is a command-line interpreter for Windows systems, to avoid detection by antivirus or file integrity monitoring tools. A rootkit can also grant the attacker remote access and control over the infected system, as well as perform malicious actions such as stealing data, installing backdoors, or launching attacks on other systems. A rootkit is one of the most difficult types of malware to remove, as it can persist even after rebooting or reinstalling the OS.

An Endpoint Detection and Response (EDR) solution is designed to monitor, detect, and respond to security incidents on endpoints (such as workstations and servers). It collects and analyzes data, detecting suspicious activity and forwarding relevant information for further investigation.