a b c d

c

Activity $ A = A_0 e^{-\lambda t}$ $A = \frac{A_0}{100}= A_0 e^{-\lambda t}$ $ \Rightarrow \lambda t = l_n100$ $ \Rightarrow t = \frac{l_n 100}{l_n 2} T = 6.6 T$