Final answer:
The Lateral decubitus position is required to effectively demonstrate small amounts of fluid in the pleural cavity on an X-ray image.
Step-by-step explanation:
To demonstrate small amounts of fluid in the pleural cavity, the optimal position is the Lateral decubitus position (A). This positioning typically involves having the patient lie on their side with the side being investigated facing upwards, which allows the fluid to layer out and become visible on an X-ray image. Alternately, for a patient unable to tolerate that position, a Cross-table lateral with a horizontal beam may be used. Positions such as AP supine (B), PA erect (C), and Right lateral recumbent (D) are not as effective in demonstrating minimal pleural effusions because fluid may be spread out or obscured in these positions.