Final answer:
In a decubitus chest x-ray, for free air detection, the affected side should be elevated, and for fluid detection, it should be dependent.
Step-by-step explanation:
When performing a decubitus chest x-ray, if free air is suspected, the affected side must be elevated, and if fluid is suspected, the affected side must be dependent. This positioning helps to determine the presence of free air or fluid in the chest. The correct answer is thus b. Dependent; elevated. This is because when the affected side is elevated, free air will rise and become more visible at the highest point in the chest cavity. Conversely, if there is a suspicion of fluid such as in cases of lung infiltrate suggestive of pneumonia, placing the affected side in a dependent position allows the fluid to settle, making it easier to detect on the x-ray.