Final answer:
The preferred Source to Image Distance (SID) for a mobile unit image of the chest is 72 inches, as this helps in reducing magnification and distortion for better diagnostic accuracy.
Step-by-step explanation:
The preferred Source to Image Distance (SID) for a mobile unit image of the chest is option D, 72 inches. This distance is chosen to reduce magnification and distortion of the chest structures, which is particularly important for accurate diagnosis.
A longer SID helps in producing a higher image quality, as it provides better resolution and reduces the effect of scattered radiation. However, in mobile or portable chest radiography, sometimes a smaller SID, such as 40 inches, may be used due to space constraints or equipment limitations, but the standard and preferred SID remains 72 inches wherever possible.