A sandbox in virtualization allows testing an application in an isolated environment to observe its behavior without affecting the host system.
Why Not A (Run an application on multiple workstations): This describes application virtualization, not sandboxing.
Why Not C (Migrate a legacy application): This is about virtualization for legacy support, not testing.
Why Not D (Create a firewall): A firewall does not act as a sandbox for application testing.
CompTIA A+ Exam Reference: Core 1 (220-1201), Section 4.2, virtualization and sandboxing.