Final answer:
In wastewater treatment, chlorine is commonly used in the form of hypochlorite solution, due to the impracticality of storing chlorine gas. Sodium hypochlorite, or household bleach, is typically used because it releases chlorine effectively when decomposed in water.
Step-by-step explanation:
When chlorine is used at a wastewater treatment plant, it is usually in the form of D) Hypochlorite solution. This is often preferred because storing poisonous chlorine gas is impractical and hazardous. Hypochlorite solutions incrementally release chlorine into the water in a safer and more manageable manner. The sodium hypochlorite form, commonly known as household bleach, is widely used due to its disinfectant properties.