An organization wants to secure the boundary between a lower-security zone and a higher-security zone. Which security measure should it use?

Host isolation refers to the practice of isolating specific hosts from one another within the same network to prevent unauthorized access. While it enhances security at the host level, it does not effectively manage or monitor traffic between different security zones. Therefore, it is insufficient for securing the boundary between zones of differing security levels.

A virtual client typically refers to a virtualized instance of a client operating system used for accessing resources remotely. While it can provide a level of security through isolation, it does not inherently act as a boundary security measure. Its primary function is to facilitate user access rather than enforce security between different zones.

Secure Shell (SSH) is a cryptographic network protocol primarily used for secure remote login and other secure network services. Although SSH provides a secure communication channel, it does not function as a boundary security measure. It is not designed to control access between different security zones, making it ineffective for this purpose.