Final answer:
Maria is the Latin word for 'seas', mistakenly thought by Galileo to be actual bodies of water on the Moon, but are in fact flat plains of basaltic lava. Option D is correct answer.
Step-by-step explanation:
Maria is the Latin word for seas, because Galileo viewed them as dark, flat surfaces on the Moon that he mistook for water. Early lunar observers categorized different aspects of the Moon's surface, with the large, dark plains being called 'maria', which is Latin for 'seas'. Giovanni Riccioli in 1651 named these features after famous astronomers and the more prominent ones after 'seas' or 'maria'. While Galileo observed many features, including craters, mountain ranges, and valleys, it was the maria that caught his attention as potential bodies of water.
The names given to these maria, such as Mare Nubium (Sea of Clouds) and Mare Tranquillitatis (Sea of Tranquility), persist to this day. However, rather than being bodies of water, we now know that these maria are actually flat plains of basaltic lava, covering about 17% of the Moon's surface. They are distinct from the highlands, which are the lighter, heavily cratered regions, elevated several kilometers higher than the maria.
The contrast between these two types of terrain is still evident in lunar observations and photography. For example, the Mare Imbrium is a vast plain of basaltic lava highlighted in images from the Apollo missions. In contrast to Galileo's initial thoughts, with today's advanced telescopes, it is evident that the lunar maria are not composed of water.
In summary, the correct answer to the question is: d. Seas.