Which of the following is a common component of a computer's power supply unit?

A cooling fan is a common component of a computer's power supply unit because the power supply generates heat due to the electrical components inside it, particularly from the conversion of AC (alternating current) to DC (direct current) power. To ensure that the power supply operates efficiently and to prolong its lifespan, it is essential to have adequate cooling. The cooling fan helps to dissipate the heat produced, maintaining optimal operating temperatures. This component is crucial in preventing overheating, which could lead to component failure or reduced performance.

In contrast, video cards, network adapters, and solid state drives are not components of the power supply unit. These components serve different functions within the computer system: video cards handle graphics processing, network adapters manage network connectivity, and solid state drives provide storage solutions. They might require power from the power supply, but they are not part of its physical structure or functionality.