Which protocol might be used to access a server that needs secure communication?

Secure Shell (SSH) is the correct choice for accessing a server that requires secure communication. SSH is specifically designed to provide a secure channel over an unsecured network by using encryption to protect the data transmitted between the client and the server. This includes not only the commands that are issued but also any sensitive information like passwords, thereby ensuring confidentiality and integrity of the communication.

Unlike other protocols mentioned, SSH is focused on secure remote access and management of systems. For example, while Post Office Protocol 3 (POP3) is used for retrieving emails, it does not inherently provide secure communication mechanisms. Similarly, Dynamic Host Configuration Protocol (DHCP) is used for dynamically assigning IP addresses to devices on a network and lacks security features. Simple Mail Transfer Protocol (SMTP) is used for sending emails but, in its basic form, does not provide secure transmission, although it can be secured using enhancements like STARTTLS.

Thus, SSH stands out as the ideal choice for secure server access among the options presented.