Final answer:
The account that should be disabled to adhere to security best practices is the Guest account because it generally lacks password protection and traceability.
Step-by-step explanation:
Adhering to security best practices, the user account that should be disabled is B. Guest. Guest accounts are typically non-secure because they don't require a password to access and can be used by anyone without traceability. In contrast, standard user accounts (A. Standard user) and administrator accounts (C. Administrator) are essential for day-to-day operations and system management respectively. However, both should have strong passwords and proper access controls. A power user (D. Power user) generally has a few more privileges than a standard user, but less than an administrator and are less common in modern systems but should also be managed securely if present.