Which of the following describes a material that is transparently thin?

The term that accurately describes a material that is transparently thin is "sheer." When something is sheer, it indicates that the material allows light to pass through, albeit in a thin, translucent manner. This quality is commonly seen in fabrics like chiffon or organza, where light can filter through while still maintaining some degree of visibility.

In contrast, the other terms do not convey the same meaning. Opaque refers to materials that do not allow light to pass through at all. Substantial implies thickness or density, suggesting the opposite of being thin and transparent. Solid also does not align with the description of being thin or transparent, as it typically refers to materials that are firm and not translucent. Therefore, sheer is the most appropriate choice to describe a material that is transparently thin.