A company uses vital legacy software that does not run in the current OS version...

Application virtualization allows legacy software to run in a virtualized environment while the operating system remains current. It isolates the application from the OS, ensuring compatibility without downgrading the OS.

Why Not A (Shared resources): This relates to resource sharing, not application compatibility.

Why Not B (System sandbox): Sandboxes isolate applications for security testing, not compatibility.

Why Not C (Test development): Test environments are used for development, not running legacy applications.

CompTIA A+ Exam Reference: Core 1 (220-1101), Section 4.2, virtualization concepts​.