Which radiation measurement unit is defined as the amount of energy absorbed by one kilogram of tissue?

The correct choice is Gy, which stands for Gray. This unit is specifically defined as the amount of energy absorbed per kilogram of matter, particularly biological tissue. In radiation therapy and radiobiology, the Gray quantifies the dose of radiation absorbed by a given mass of tissue, which is essential for assessing the potential biological effects of radiation exposure.

The concept of the Gray allows clinicians and medical physicists to accurately measure and compare doses in treatment planning for patients undergoing radiation therapy. It provides a standardized way to express the quantity of radiation absorbed, facilitating communication among healthcare professionals and ensuring consistency in patient care.

Units like REM and RAD serve specific purposes in the context of radiation exposure and biological effects, but they do not measure the absorbed dose in the way that the Gray does. The Roentgen is also a measure of exposure to ionizing radiation, but it is not equivalent to the absorbed dose directly related to tissue energy absorption. Thus, the definition of the Gray makes it the most appropriate choice for this question.