Which of the following describes the composition of conglomerate?

Conglomerate is a type of sedimentary rock that is primarily characterized by its composition of larger clasts, particularly rounded fragments of rock or minerals that are cemented together by finer material, which can include sand and silt. The definition of conglomerate includes cobbles and gravel, which are larger than sand grains and contribute to the rock's distinctive texture. The presence of these various clast sizes, along with the matrix that binds them, is crucial in classifying a rock as conglomerate.

In contrast, fine-grained sandstone would indicate a rock composed mainly of very small sand grains, lacking the larger clasts characteristic of conglomerates. A completely crystalline structure suggests a different type of rock entirely, possibly an igneous or metamorphic rock, rather than a sedimentary one like conglomerate. Lastly, purely volcanic material would pertain to volcanic rock types, which do not align with the composition of conglomerate. Thus, the description of conglomerate as composed of cemented cobbles, gravel, and sand aligns accurately with the defining characteristics of the rock.