a b c d

a

$ n = \frac{t}{t_1/2} = 4$ $ N = \frac{N_0}{2^n} = \frac{N_0}{16}$ $\text{Amount of substance decayed is } \frac{15N_0}{16}$ $ \text{Percentage decayed } \frac{15N_0/16}{N_0} \times 100 = 93.75$%