What is the unit used to measure the amount of radiation energy absorbed by material?

The correct choice is Radiation Absorbed Dose (RAD), which is a standard unit used to quantify the amount of ionizing radiation energy absorbed by a material, including human tissue. One RAD is defined as the absorption of 100 ergs of energy per gram of absorbing material. This unit is fundamental in radiation protection and medical physics, helping to assess the potential damage caused by exposure to ionizing radiation.

Other units mentioned, such as Rem or Millirem, are used for measuring dose equivalents, which take into account the biological effect of the radiation type on living tissue. They are related to the concept of radiation risk rather than direct energy absorption. Microrem is simply a smaller subdivision of Rem, making it suitable for measuring low levels of radiation exposure, but it too focuses on dose equivalent rather than absorbed energy directly.