Final answer:
The dark regions of the moon, known as the lunar maria, are primarily composed of basalt option (a), a volcanic rock. These areas were formed from ancient volcanic lava flows and their composition is akin to Earth's oceanic crust.
Step-by-step explanation:
The most common rock type in the dark regions of the moon, known as the lunar maria, is basalt. These dark-colored volcanic areas were formed from lava during massive volcanic eruptions billions of years ago.
The basaltic composition is similar to that found in the Earth's oceanic crust and the lavas produced by many terrestrial volcanoes. Over time, these basalt lava flows filled the large depressions created by impacts, resulting in the flat plains we now observe on the Moon's surface.
The lunar basalt has unique features, such as the holes left by gas bubbles and a chemical distinctness that allows scientists to identify lunar rocks amongst Earth's meteorites. Additionally, the surface of the Moon is covered by fine-grained soil formed from countless impacts that have pulverized the rock into tiny fragments, often the size of dust or sand.