Algae with two shells that overlap made up of silica

Explanation

The correct answer is Diatoms. Diatoms are a group of algae that have a unique cell wall made of silica, forming two overlapping shells or frustules. This silica-based structure is a key characteristic that distinguishes diatoms from other types of algae. Dinoflagellates have cellulose plates, not silica shells. Brown algae are large and multicellular with no silica shells, and red algae are noted for their red pigments and also lack silica shells.