After first half hrs $N=N_{0} \frac{1}{2}$

for $t=\frac{1}{2}$ to $t=1 \frac{1}{2}$

$N=\left(N_{0} \frac{1}{2}\right)\left[\frac{1}{2}\right]^{4}=N_{0}\left(\frac{1}{2}\right)^{5}$

For $t=1 \frac{1}{2}$ to $t=2$ hrs.

[for both $A$ and $B \frac{1}{t_{1 / 2}}=\frac{1}{1 / 2}+\frac{1}{1 / 4}$

$=2+4=6$

$t_{1 / 2}=1 / 6 \text { hrs.] }$

$N=\left[N_{0}\left(\frac{1}{2}\right)^{5}\right]\left(\frac{1}{2}\right)^{3}=N_{0}\left(\frac{1}{2}\right)^{8}$

So, The correct option of this question is **(A).**