Final answer:
An action that cannot be done to an existing user is reusing the sandbox login, as sandboxes are usually temporary testing environments distinct from regular user account actions.
Step-by-step explanation:
The question is asking which action cannot be taken with respect to an existing user account in a computer system or network. The answer is A. Reuse the sandbox login. Typically, a sandbox is an isolated testing environment that enables users to run programs or open files without affecting the application, system, or platform on which they are running. Sandboxes can be temporary and are not typically associated with actions such as resetting a password, deleting a user, or changing a user role, which are standard functions that can be performed on user accounts in most systems.
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