A technician upgrades a CPU heat sink with a higher-performing model.

Whenever a CPU heat sink is removed or replaced, thermal paste must be reapplied. Thermal paste fills microscopic gaps between the CPU heat spreader and the heat sink, ensuring efficient heat transfer. Without it, there is a significant air gap, which acts as an insulator, causing CPU temperatures to rise dramatically even under light load. CompTIA A+ stresses thermal compound application as a key step in CPU cooling installation.

The scenario indicates that temperatures increased drastically after upgrading to a better heat sink. A higher-performing heat sink should improve temperatures, not worsen them—unless a critical installation step was missed. Missing thermal paste is the most common cause of this symptom and easily explains the sudden overheating.

Decreased CPU voltage would lower temperatures, not raise them. Higher fan RPM increases cooling efficiency, not reduces it. Airflow interruption is possible but less likely in a simple heat sink upgrade scenario unless the technician installed the CPU cooler incorrectly; however, the most direct and common cause described by CompTIA is missing thermal paste.

Thus, D is the correct answer.