To enforce key-based authentication for SSH and disable the use of usernames and passwords, the systems administrator should do the following:

**A. Instruct users on how to create a public/private key pair and install users' public keys on the server.**

This is the correct next step. In this approach, users are responsible for generating their own key pairs, and the public keys are added to the server's authorized keys file. It's a standard and secure way to enforce key-based authentication, and it allows users to manage their own keys, which can be revoked or rotated as needed.

Option B, changing the default SSH port, enabling TCP tunneling, and providing a pre-configured SSH client, is not directly related to enforcing key-based authentication and doesn't address the primary goal of disabling password authentication.

Option C, issuing public/private key pairs for each user and distributing private keys, can be administratively burdensome and may not align with best practices for key management. Users typically generate their own key pairs for security reasons.

Option D, disabling username and password authentication and enabling TOTP (Time-based One-Time Password) in the sshd.conf file, is a different authentication mechanism that doesn't enforce key-based authentication, which is the specific requirement in the question....