Which of the following types of ceilings indicates that a significant portion of the sky is covered?

The choice indicating that a significant portion of the sky is covered is represented by "OVC," which stands for overcast. This term specifically denotes conditions where the sky is entirely covered by clouds, meaning there are no breaks or clear areas visible. In aviation and meteorological contexts, "OVC" is important as it indicates limited visibility and can affect flight operations, leading to conditions that require instruments for navigation.

Other terms have different meanings related to cloud coverage. For example, "VV," or vertical visibility, indicates how far one can see vertically into a cloud but does not specifically reference sky coverage. "BKN," or broken clouds, refers to when 5/8 to 7/8 of the sky is covered, indicating significant cloud presence but not complete coverage. "CIG," or ceiling, refers to the height of the lowest layer of clouds above the ground but does not indicate the proportion of the sky that is covered. Therefore, "OVC" is the ideal choice for describing a situation where a significant portion of the sky is obscured by clouds.