Which of the following is considered a secure service as required for system functionality?

The choice of SSH as the secure service required for system functionality is grounded in its strong encryption and authentication capabilities. SSH, or Secure Shell, is widely used for secure remote management of servers and systems. It provides a secure channel over an unsecured network in a client-server architecture, effectively protecting the data transmitted between the two endpoints.

In contrast, the other options like FTP, HTTP, and Telnet do not offer the same level of security. FTP (File Transfer Protocol) and HTTP (Hypertext Transfer Protocol) transmit data in unencrypted form, making them susceptible to eavesdropping and attacks. Telnet, similarly, is an unencrypted protocol that allows users to access remote computers but does so without any form of secure communication. This vulnerability can lead to sensitive information, such as passwords and data, being intercepted by malicious actors.

In a compliance context, such as under PCI DSS requirements, using secure services like SSH is crucial to ensuring the confidentiality and integrity of sensitive data during transmission, thereby helping organizations maintain secure systems and protect against potential data breaches.