Zach, a user, cannot access any network resources. The technician has determined that the problem is isolated to Zach, and that there have not been any changes to his workstation or switch port recently. Which of the following steps in the troubleshooting methodology should the technician do NEXT?

A.
Establish a plan of action to resolve the problem.

B.
Implement preventative measures.

C.
Implement the solution or escalate as necessary.

D.
Establish a theory of probable cause.