Which of the following is considered an irritant?

Chlorine is considered an irritant due to its highly reactive nature and its potential to cause irritation to the eyes, skin, and respiratory tract when inhaled or contacted. It is commonly used as a disinfectant in swimming pools and water treatment, but exposure to chlorine can lead to a range of symptoms, including coughing, throat irritation, and eye discomfort. This irritant property is primarily due to the chemical's strong oxidizing ability, which can react with moisture in the lungs to form hydrochloric acid, resulting in irritation and inflammation.

In contrast, water is essential for life and usually does not cause irritation under normal circumstances. Oxygen, while vital for respiration, does not have irritating properties at typical environmental concentrations. Nitrogen, which makes up a large portion of the Earth's atmosphere, is inert and does not usually cause irritation either. Thus, chlorine stands out as the substance with a recognized potential to irritate human tissues, making it the correct choice in this context.