What type of rock consists of well-rounded quartz grains?

The correct choice is sandstone. Sandstone is primarily composed of sand-sized particles, most commonly quartz grains, which can be well-rounded due to the processes of weathering and erosion. This rounding occurs as the grains are transported by wind or water, allowing them to collide with each other and with other materials, smoothing their edges and giving them a rounded appearance.

In the context of sedimentary rocks, sandstone is characterized by its medium grain size, which is larger than siltstone but smaller than conglomerate. The composition of sandstone can vary, but when it is predominantly made up of quartz, it indicates a significant amount of geological processes that have brought those grains together in a cohesive form.

Conglomerate, while also a type of sedimentary rock, differs because it consists of larger gravel-sized particles that are typically more angular or less well-rounded compared to the grains found in sandstone. Additionally, siltstone consists of finer particles that are not large enough to be categorized as sand, and limestone is mainly composed of calcium carbonate and is typically formed from biological processes rather than being composed of quartz grains. Hence, sandstone stands out as the correct answer due to its well-rounded quartz grain composition.