**Answer:**

The kaolinite structural unit is made up of a gibbsite sheet.

**Explanation:**

Kaolinite is a clay mineral that belongs to the kaolin group of minerals. It is a layered silicate mineral, meaning that it is made up of layers or sheets of atoms. The structural unit of kaolinite consists of one silica tetrahedral sheet bonded to one gibbsite octahedral sheet.

- **Silica Tetrahedral Sheet:** The silica tetrahedral sheet is composed of silicon (Si) atoms surrounded by four oxygen (O) atoms, forming a tetrahedral arrangement. The oxygen atoms are shared between adjacent tetrahedra, creating a continuous sheet of silica tetrahedra.

- **Gibbsite Octahedral Sheet:** The gibbsite octahedral sheet is composed of aluminum (Al) atoms surrounded by six hydroxide (OH) ions, forming an octahedral arrangement. The hydroxide ions are shared between adjacent octahedra, creating a continuous sheet of gibbsite octahedra.

In the kaolinite structural unit, the silica tetrahedral sheet and the gibbsite octahedral sheet are stacked on top of each other, with the oxygen atoms of the silica tetrahedral sheet bonding to the hydroxide ions of the gibbsite octahedral sheet. This bonding between the two sheets creates a strong and stable structure.

The presence of the gibbsite sheet in the kaolinite structure gives it certain properties and characteristics. The gibbsite sheet contributes to the overall charge balance of the kaolinite mineral, as the aluminum atoms in the gibbsite sheet have a positive charge that is balanced by the negative charge of the silica tetrahedral sheet. The gibbsite sheet also provides sites for cation exchange and adsorption, allowing kaolinite to interact with other substances in its environment.

Therefore, the correct answer is option A: Gibbsite sheet.