**Core Concept**
The outer covering of diatoms, a type of algae, is composed of a unique material that provides protection and support. This material is made of silica, which is deposited in a specific pattern to form a rigid, glass-like structure called frustule.

**Why the Correct Answer is Right**
The frustule of diatoms is indeed made of silica, which is secreted by the diatom's cell membrane. The silica is deposited in a specific pattern to form the characteristic shape of the diatom, including the two halves of the frustule that fit together like a lid and a box. The silica frustule provides protection to the diatom's cell from mechanical damage and also helps to maintain its shape.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because diatoms are not covered by a lipid bilayer, which is a characteristic of cell membranes in eukaryotic cells.
* **Option B:** This option is incorrect because cellulose is a component of plant cell walls, not diatom frustules.
* **Option C:** This option is incorrect because chitin is a component of insect exoskeletons and fungal cell walls, not diatom frustules.

**Clinical Pearl / High-Yield Fact**
Diatoms are important organisms in aquatic ecosystems, and their frustules can be used as a proxy for monitoring water quality and climate change.

**Correct Answer: D. Silica. The outer covering of diatoms is made of silica.**