Which of the following factors is a significant contributor to visibility reduction?

Cloud cover is indeed a significant contributor to visibility reduction because it can lead to various atmospheric conditions that obscure vision. When clouds are thick and low-hanging, they can directly block a pilot's line of sight to the ground, other aircraft, or navigational aids. Additionally, clouds can be associated with precipitation, such as rain, snow, or fog, further reducing visibility.

While high temperatures can affect several weather phenomena, they typically do not have a direct impact on visibility the way cloud cover does. Similarly, calm winds might not create conditions conducive to visibility reduction; instead, they can lead to the accumulation of fog under certain circumstances, but this is not as critical as the impact of cloud cover. High altitudes also do not inherently reduce visibility but may enhance the distance from which one can see if one is above the cloud cover. Thus, cloud cover stands out as a primary factor in visibility reduction.