Final answer:
The output from an X-ray tube is measured in Roentgens (R), which quantify radiation that produces an electrical charge in dry air.
Step-by-step explanation:
The radiation output from a diagnostic X-ray tube is primarily measured in the unit Roentgen (R). The Roentgen measures the amount of radiation that produces an electrical charge in a specific quantity of dry air, which can be used to quantify exposure. However, it is essential to note that for the effects of radiation on biological tissue, units such as rad (radiation absorbed dose) and rem (roentgen equivalent man) are commonly used in medicine. The rad quantifies energy absorbed by any material, while the rem accounts for energy absorption along with the relative biological effects on human tissue. In summary, the correct answer to the question is C, Roentgen.