Which of the following describes absorbed dose?

The absorbed dose is defined as the amount of energy deposited by ionizing radiation in a given mass of tissue. This means it quantifies the energy absorbed per unit mass, making energy per mass the correct characterization of absorbed dose. The absorbed dose is usually expressed in units called grays (Gy), where one gray corresponds to an absorption of one joule of radiation energy per kilogram of matter.

The other options do not accurately describe absorbed dose. Energy per unit area pertains more to concepts like energy flux or irradiance, which are related to how energy spreads over a surface area. Energy per time refers to power, which indicates the rate at which energy is being used or transferred, instead of how much energy is absorbed. Energy over a lifetime might indicate cumulative energy but lacks the specificity regarding mass that absorbed dose defines. Thus, energy per mass provides a precise measure that is directly relevant to understanding the biological effects of radiation, underscoring the importance of the correct answer.