as radioactivity follow first order kinetics
given half life =14.26days
as we know for first order reaction
half life =0.693/K
then K =rate constant =0.693/half life =0.693/14.26
as K for first order is
K =(1/t)* ln(A°/A). ...... (1)
lets original activity =A° =100
then A =100-18.3 =81.7
now put in (1)
K =(1/t) ln(100/81.7) =0.693/14.26
from here t =(14.26/0.693)*ln(100/81.7) =4.2 days
thus second option is correct.
based on calculation, other options are incorrect.
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