**Solution —**

Moles of $D_{2} O=\frac{4}{20}=\frac{1}{5}$

No. of neutrons in $D=1$ and in oxygen $=8$

So, total no. of neutrons in 1 mole $D_{2} O=10 \mathrm{~N}_{A}$

Now, in $\frac{1}{5}$ moles $D_{2}O$, no. of neutrons $=10 \mathrm{~N}_{\mathrm{A}} \times \frac{1}{5}=2 \mathrm{~N}_{\mathrm{A}}$

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