The length of time required for half of the radioactive atoms in a sample to decay is its A. half-life. B. relative date. C. radioactive date. D. period.

A half life is the time it takes for half of a certain type of atoms to decay :)

The correct answer is half-life. The half life defines the minimum amount of time it will take for a particle to reach the point in the decay process in which it has experienced half the time of it's expectancy.