Final answer:
The clinically significant gas on a chest radiograph located under the right hemidiaphragm is accurately described in option B. This finding often suggests a surgical emergency due to the presence of free air from a perforated abdominal viscus.
Step-by-step explanation:
If the clinically significant gas is located under the right hemidiaphragm in a chest radiograph, the correct answer is B. Under the right hemidiaphragm. Gas under the diaphragm can be a sign of a perforated viscus, such as when an ulcer perforates or following abdominal surgery, and usually indicates the presence of free air in the abdominal cavity that has risen to the highest point, which is under the diaphragm when a person is lying down. These findings are often associated with a surgical emergency. However, it is important to note that the presence of gas within the stomach is normal and would be visible in the stomach area on an X-ray, which is option A.