Solution

Cell of diatoms are surrounded by a silica-based shell/cell wall called as frustules which is composed of two overlapping halves (the epitheca and the hypotheca). Presence of numerous tiny pores in frustules connects the protoplast of the cell with the outside environment. Calcium carbonate is present in cell wall of red algae and pellicle of protozoans; it makes the cell permeable to small molecules but impermeable to macromolecules. Red algae secrete calcium carbonate shells which are accumulated in sea bottom and forms red corals. Keratin is an $$\alpha$$-helix protein and is present in skin, hair or nails of animals where it is acted upon by keratinase enzyme of dermatophytes, the fungi that cause cutaneous mycoses. Calcium oxalate is poisonous, even fatal substance which is the main constituent of human kidney stone; it is not present in cell wall. Thus, the correct answer is option A.