Final answer:
To resolve the issue of network administrators not being prompted for the second factor of authentication, the network devices should be reconfigured to prompt for username, password, and a token.
Step-by-step explanation:
The issue described in the question is that the network administrators are not being prompted for the second factor of authentication when trying to log onto the network devices. To resolve this issue, option C should be implemented, which is to reconfigure the network devices to prompt for username, password, and a token. This means that the network administrators will enter their username and password, and then they will enter the token. This ensures that two-factor authentication is properly enforced and the administrators are prompted for both factors.