Which of the following is not advised to be present with a GE Lunar Machine?

The presence of air with a GE Lunar Machine is not advised because air can introduce variability and inaccuracies in the measurements of bone density. The GE Lunar Machine functions by using dual-energy X-ray absorptiometry (DXA) to measure bone mineral density. The presence of air can lead to scatter and attenuation of the X-ray beams, which may compromise the quality and reliability of the results.

In contrast, oxygen, carbon dioxide, and argon are gases that can be present without significantly impacting the machine's ability to take accurate measurements. These gases can exist in the background atmosphere without affecting the X-ray transmission through the bone. Therefore, it is the presence of air that poses concerns for the precision needed in bone density assessments using the GE Lunar Machine.