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)