Final answer:
To diagnose a left-sided pneumothorax with decubitus imaging, the patient should be placed in the left lateral decubitus position to allow clear visualization on a chest radiograph.
Step-by-step explanation:
For a patient requiring decubitus imaging to diagnose a left-sided pneumothorax, the correct position would be b. Left lateral decubitus. In this position, the patient lies on their left side, which is the affected side in this case, allowing free air in the pleural space to rise to the highest point and create a visible difference between the air and the lung on the radiograph. This position helps in confirming the presence of pneumothorax by allowing clear visualization of the pleural line separate from the lung margin.