The correct answer is option 'D': silica.

Diatoms and radiolaria are microscopic marine organisms that are part of the plankton community. They are single-celled organisms that have a protective outer shell, or test, made of various materials. These shells accumulate on the abyssal plain, which is the flat and sediment-covered seafloor found in the deepest parts of the ocean.

1. Diatoms and Radiolaria:
- Diatoms are a type of phytoplankton, which are photosynthetic microorganisms that form the base of the marine food chain.
- Radiolaria are zooplankton, which are heterotrophic organisms that consume other organisms or organic material.
- Both diatoms and radiolaria play important roles in marine ecosystems, as they are a food source for many larger organisms and help in carbon fixation.

2. Composition of Shells:
- The shells, or tests, of diatoms and radiolaria are made of different materials depending on the species.
- Diatoms have shells composed of silica, specifically hydrated silicon dioxide (SiO2·nH2O).
- Radiolaria also have shells made of silica, but they often have intricate and delicate structures due to their radial symmetry.

3. Silica:
- Silica is a compound that is abundant in the Earth's crust and can be found in various forms, such as quartz and sand.
- It is a hard and chemically stable material, making it suitable for the protective shells of diatoms and radiolaria.
- Silica is produced by these organisms through the uptake of dissolved silica from the surrounding seawater, which is then used to build their shells.

4. Accumulation on the Abyssal Plain:
- The abyssal plain is a vast, flat area of the seafloor found in the deepest parts of the ocean.
- The shells of diatoms and radiolaria sink to the bottom of the ocean after the organisms die, creating a layer of sediment on the abyssal plain.
- Over time, this accumulation of shells and other organic matter forms a thick layer of fine-grained sediment known as ooze.

In conclusion, the shells of diatoms and radiolaria, which accumulate on the abyssal plain, are composed of silica. Silica is a hard and chemically stable material that is produced by these organisms and provides protection for their single-celled bodies.