Which unit is the unit of activity?

• A. curie
• B. sievert
• C. becquerel
• D. gray

Answer: (C)

Activity is the rate at which a radioactive sample undergoes decays, i.e., disintegrations per unit time. The standard SI unit for this rate is the becquerel, defined as one disintegration per second. This makes it a direct measure of how actively a source is decaying at that moment. The curie is an older, non-SI unit that equals about 3.7 × 10^10 disintegrations per second, still used in some contexts but not SI. The gray and the sievert describe energy deposited and biological effect, not how fast decays occur. So the unit that directly measures activity—the rate of decays—is the becquerel.