The contraction used to describe a total ground-based obscuration is:

The correct choice is indicated by the contraction "VV," which specifically refers to vertical visibility in the context of aviation weather observations. This notation is used when there is a complete obscuration of the horizon, usually due to fog, haze, or heavy precipitation, making it impossible to see the ground or horizon at all.

"VV" is derived from the term "vertical visibility," signifying that the visibility measured vertically from the observer is limited to a specific distance, often in feet. In scenarios of total ground-based obscuration, vertical visibility provides critical information to pilots about the limits of their visibility—and thereby helps them make informed decisions regarding their flight.

In contrast, the other options do not adequately represent total ground-based obscuration. For example, the contraction "W" represents "warning," while "X" typically denotes "unknown," and "SKC" means "sky clear." Each of these terms has distinct meanings unrelated to total obscuration, making "VV" the appropriate and precise choice in this context.