A curie is a unit for measuring what?

A curie is indeed a unit that measures radioactivity. Specifically, it quantifies the amount of radioactive decay occurring in a sample. One curie is defined as the amount of a radioactive material that undergoes 37 billion disintegrations per second, reflecting the activity of a radioactive source.

The other choices relate to different aspects of radiation measurement. Radiation absorbed dose measures the energy deposited by ionizing radiation in a material, typically expressed in grays or rads. Dose equivalence accounts for the biological effect of different types of radiation, using units such as sieverts or rems. Lastly, gamma or x-ray exposure in dry air is measured in units like roentgens, focusing on the amount of ionization produced in air by x-ray or gamma radiation. Thus, these choices represent distinct concepts in radiation measurement, while the curie specifically defines the activity level of radioactive materials.