Which of the following is not a pollutant? 

• Hydrogen is not one of the pollutant.
• Hydrogen is known as a cleaner fuel.
• CO_2, SO₂ and NO₂  are the air pollutants.
• Substances or energies that are introduced into the natural environment and cause unintended alterations are referred to as pollutants.
• Pollutants are any sorts of foreign substances that have an unfavorable impact on the ecosystem. 
• They disrupt the typical atmospheric circumstances.

Hence, option C is the correct answer.