Limestone, chalk and marble are different forms of ______.

calcium carbonate

The correct answer is option 1. calcium carbonate.

Limestone, chalk, and marble are all types of sedimentary rocks that are composed primarily of calcium carbonate \((CaCO_3)\).

1. Limestone: This sedimentary rock forms from the accumulation of marine organisms such as shells, coral, and algae over millions of years. These organisms contain calcium carbonate in their shells or skeletons. When they die, their remains settle on the ocean floor and undergo compaction and cementation to form limestone.

2. Chalk: Chalk is a soft, white, porous sedimentary rock that also consists mainly of calcium carbonate. It forms from the accumulation of microscopic planktonic algae called coccolithophores, which secrete calcium carbonate shells. Over time, these shells accumulate on the ocean floor and are compacted to form chalk deposits.

3. Marble:  Marble is a metamorphic rock that forms from the recrystallization of limestone under heat and pressure deep within the Earth's crust. During metamorphism, the original calcite grains in limestone undergo recrystallization and reorganization to form larger, interlocking crystals of calcite or dolomite, resulting in the characteristic texture and appearance of marble.

In summary, all three types of rocks share the common mineral composition of calcium carbonate, but they differ in their formation processes and physical properties due to differences in their geological history and subsequent environmental conditions.