Which of the following should be done before placing a server on a network?

Before placing a server on a network, it is crucial to harden the operating system (OS). This process involves applying security measures to reduce vulnerabilities and improve the server's defenses against potential attacks. Hardening can include steps such as applying the latest security patches, disabling unnecessary services, configuring firewalls, and setting secure password policies. These actions are essential to protect the server from threats that could exploit weaknesses in the OS once it is connected to the network.

Securing the disk and securing the server rack are important but typically relate to physical and data security rather than the immediate network implications. Additionally, while installing applications is a necessary step in setting up a server for specific tasks, it should follow the essential hardening process. Placing an unsecured server on the network could expose it to a range of security risks, making hardening a priority to ensure its integrity and safety in a networked environment.