Ans is (c)
frequency : the fact or state of being frequent or happening often. frequency is constant over the time interval.
periodicity : tendency to occur at interval. its somewhat similar to frequency the only difference is periodicity
means happening once over the time but frequency means happening more number of times within an interval. In other word frequency is one over the time interval.
rarity : the state or quality of being rare. somewhat frequency is 0.
gradualness : Occurring slowly or by small increments. It means frequency of occurrence will be decreasing over the time.
persistence : the ability to stick with something.