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).