a b c d

c

$\text{ For Nuclei A } n_1 = \frac{t}{t_1/2} = 4$ $\text{ For Nuclei B } n_2 = \frac{t}{t_1/2} = 2$ $ N_A = \frac{N_0}{2^{n_1}} = \frac{N_0}{2^4} = \frac{N_0}{16}$ $ N_B = \frac{N_0}{2^{n_2}} = \frac{N_0}{2^2} = \frac{N_0}{4}$ $ \frac{N_A}{N_B} = 1:4$