The technician has identified the symptoms (slow performance and random pop-ups) and must now establish a theory of probable cause, such as malware or spyware infection. This step aligns with the CompTIA troubleshooting methodology.
Why Not A (Establish a knowledge base): This is irrelevant at this stage; knowledge bases are used for reference and documentation.
Why Not C (Establish a new theory and consider escalating): The technician has not yet created or tested a theory, making escalation premature.
Why Not D (Establish a plan of action): Planning an action occurs after determining the cause of the problem.
CompTIA A+ Exam Reference: Core 2 (220-1102), Section 3.1, troubleshooting methodology.