If there are muons at time , then at some later time , there would be
So, at time , the muons decay at a rate
So, in a time interval ,
muons decay. So, the probability that a muon decays in this time interval is
A muon that travels a distance in the laboratory will decay at time in its rest frame.
So, . So, the probability that a muon decays after traveling a distance is
The mean distance traveled by the muons is therefore,
Plugging in and gives
Therefore, answer (C) is correct.