Becquerel is the unit of:

Explanation

The Becquerel (Bq) is the standard unit for measuring radioactivity, specifically the activity of a radioactive substance. It quantifies the number of radioactive decays per second, making it a measure of the substance's activity. This is distinct from the decay constant, half-life, and mean life, which relate to probabilities and time intervals rather than direct decay rates.Option A, the decay constant, is a probability measure indicating decay likelihood; Option B, half-life, is a time measure for decay processes; and Option C, mean life, is an average time measure for decay. Only Option D, activity, correctly identifies what is measured by Becquerels, highlighting the rate of decay events per second.