Correct option is (a) Enhanced biological phosphorus removal
Easy explanation: Sewage treatment operations in a plant to remove phosphate in an activated sludge system is called as Enhanced biological phosphorus removal in absence of nitrate and oxygen prior to aeration, Enhanced potassium removal is not preferred in an activated sludge, enhanced biological nitrogen removal is done through multiple nitrification and denitrification stages, aeration process is called as enhanced nitrogen removal process.