The correct option is A 12.65 days
We know that K=0693t12=0.6933.8=0.182 day−1
Let the initial amount of radon be N_0 and the amount left after t days be N which is equal to N010
Applying the equation,
t=2.303KlogN0Nt=2.3030.182logN0N010=2.3030.182log 10=12.65 days