The correct option is D Mycoplasma pneumoniae
Primary atypical pneumonia refers to pneumonia caused by certain bacteria, including Legionella pneumophila, Mycoplasma pneumoniae, and Chlamydophila pneumoniae. It is called as atypical because the symptoms differ from those of pneumonia due to other common bacteria. This type of pneumonia is also sometimes referred to as walking pneumonia, in reference to the fact that it is less severe than other forms of lung infection. Thus, option D is correct.