Which of the following is/are the form(s) of calcium carbonate (CaCO₃)?

All of these

The correct answer is option 4. All of these.

All of the options provided—limestone, chalk, and marble—are different forms of calcium carbonate (\( \text{CaCO}_3 \)), a chemical compound commonly found in nature.

1. Limestone: Limestone is a sedimentary rock composed primarily of calcium carbonate. It forms from the accumulation and compaction of marine organisms such as coral, shells, and skeletal fragments over millions of years. Limestone is widely used in construction, as a building material, in the manufacture of cement, and in various industrial applications.

2. Chalk: Chalk is a soft, white, porous sedimentary rock composed mainly of tiny shell fragments of marine organisms called coccolithophores. These microscopic algae produce calcareous plates made of calcium carbonate, which accumulate on the ocean floor and form chalk deposits. Chalk is commonly used for writing, drawing, and as a filler material in various industries.

3. Marble: Marble is a metamorphic rock formed from the recrystallization of limestone under high pressure and temperature conditions deep within the Earth's crust. It is primarily composed of calcite or dolomite crystals, both of which are forms of calcium carbonate. Marble is prized for its beauty and is commonly used in sculpture, architecture, and interior decoration.

In summary, limestone, chalk, and marble are all different forms of calcium carbonate, each with its own unique properties and uses. Despite their differences in texture, appearance, and formation process, they all share the common chemical composition of calcium carbonate (\( \text{CaCO}_3 \)).