Which of the following is a characteristic of lunar maria?

Lunar maria are large, dark, basaltic plains that are formed by ancient volcanic eruptions on the Moon’s surface. These plains result from lava flows that filled large impact basins, creating smooth and relatively flat areas when compared to the surrounding highlands. The term "maria" itself is derived from the Latin word for "seas," as early astronomers mistakenly believed these dark areas were actual oceans.

The option indicating lava plains accurately describes the characteristics of lunar maria because they are primarily made up of basaltic rock, which is formed from the solidification of lava. These regions are less cratered than the highlands because they are younger geological features, indicating they were formed after many of the impact craters on the Moon's surface.

In contrast, deep water-filled basins do not characterize the Moon, as there is no significant presence of liquid water. Rocky craters are prevalent on the lunar surface, particularly in the highlands, but they do not describe the smooth, flat nature of the maria. Lastly, snowy surfaces do not exist on the Moon; the extreme temperature fluctuations and lack of atmosphere prevent the formation of snow. Thus, the option focused on lava plains captures the defining aspect of lunar maria accurately.