Explanation:

- Option 1: Sulphate

- Sulphates are salts or esters of sulfuric acid containing the anion SO4²?.

- They do not contain aluminium or fluorine as main elements.

- Option 2: Phosphate

- Phosphates are salts or esters of phosphoric acid containing PO4³?.

- Not related to aluminium or fluorine structure in topaz.

- Option 3: Carbonate

- Carbonates are salts of carbonic acid containing the anion CO3²?.

- They do not form main components in topaz.

- Option 4: Silicate

- Silicates are minerals containing silicon and oxygen, and may include other elements.

- Topaz is not primarily a silicate; it is a silicate mineral with an aluminium and fluorine-based structure.

- Correct Answer: Topaz is a silicate mineral with a structure based on aluminium and fluorine.