Which of the following is NOT a chemical coagulant used in water treatment?

Nitrogen dioxide

The correct answer is option 4. Nitrogen dioxide.

Nitrogen dioxide is NOT a chemical coagulant used in water treatment. Nitrogen dioxide is a gas and is not typically used as a coagulant in water treatment processes. The other options, Polyaluminium chloride (PAC), Aluminium sulfate (Alum), and Aluminium chloride, are common chemical coagulants used for water treatment to help remove impurities and particles.