Answer
b. silica
Work Step by Step
The silica shell secreted by the diatoms is actually made up of two shells that come together and overlap one another, one fitting inside the other like a box and lid. The silica shell made by radiolaria consists of two layers, the inner layer enclosing the usual organelles found in eukaryotes and the outer layer containing vacuoles filled with gas to keep the organism afloat in water.
