Which of the following is NOT a form of chlorine?

Calcium carbonate is not a form of chlorine, which makes it the correct choice for this question. Chlorine exists in several forms, specifically as gas, liquid, and solid, depending on its chemical compounds. Pure chlorine is typically found as a gas (Cl2), while sodium hypochlorite is a common liquid disinfectant used in water treatment processes. Additionally, calcium hypochlorite is a solid compound that is widely used for water chlorination. In contrast, calcium carbonate is primarily used as a calcium supplement or antacid and does not contain chlorine, indicating it does not serve any disinfection or chlorination purposes. Hence, it does not belong in the category of chlorine forms.