Which of the following will help dissipate heat through the heat sink when upgrading a CPU?

A case fan improves general chassis airflow but does not directly address the thermal interface resistance between the CPU die and the heat sink contact surface.

Thermal compound (thermal paste or TIM - thermal interface material) is applied between the CPU heat spreader and the heat sink base to eliminate air gaps that would otherwise act as thermal insulators. By ensuring full surface contact, it dramatically improves heat transfer efficiency from the processor to the heat sink, preventing thermal throttling and CPU damage.

A motherboard riser (standoff) is a fastener used to mount the motherboard to the chassis and plays no role in CPU thermal management.

A power supply converts AC to DC voltage for system components and has no function in transferring or dissipating heat from the CPU.