Which remote desktop software is proprietary and supports multi-session environments?

The correct choice is NX because it is a proprietary remote desktop software that is specifically designed to work in multi-session environments. NX offers high-performance remote desktop access by employing compression and caching mechanisms, making it efficient even over low-bandwidth connections. It allows multiple users to connect to the same machine simultaneously, with each session maintained independently, which is essential for environments where several users may need to access resources concurrently.

In contrast, SPICE is typically associated with virtualization environments and focuses on providing improved graphics performance for virtual machines. X Forwarding, often used in Unix-like systems, allows graphical applications to be run remotely but does not inherently support multi-session capabilities in the way NX does. Systemd, on the other hand, is an init system and service manager for Linux operating systems and is not relevant to remote desktop software.

Thus, NX stands out as the solution that meets the criteria of being proprietary and supporting multi-session environments.