What is diatomaceous earth primarily composed of?

Diatomaceous earth is primarily composed of the fossilized remains of diatoms, which are a type of microscopic organism. These organisms have a silica-based cell wall, known as a frustule, that provides structure and support. Over time, as diatoms die and accumulate in sedimentary environments, they form diatomaceous earth, which is rich in silicon dioxide. This makes diatomaceous earth a unique material that is often used for various applications, such as filtration, pest control, and as a mild abrasive in products.

While silicon is a significant component of diatomaceous earth due to its silica content, the defining characteristic of diatomaceous earth is the accumulated remains of diatoms. Other options like calcium carbonate and organic material from plants do not accurately describe the composition of diatomaceous earth. Thus, the focus on the shells of these microscopic organisms as the primary composition is what makes the answer correct.