The number of neutrons in a drop of water (20 drops= $1\text{mL}$) at $4^{\circ }\text{C}$ is __________.

The correct option is B $1.338\times {10}^{22}$
20 drops = $1\text{mL}$
So, 1 drop = $\frac{1}{20}\text{mL}$
$\text{Mass of one drop}=\text{Volume}\times \text{density}$
$=\frac{1}{20}\text{mL}\times 1\frac{\text{g}}{\text{mL}}=\frac{1}{20}\text{g}$
Number of moles $=\frac{\frac{1}{20}}{18}=\frac{1}{360}$
Number of molecules $=\frac{1}{360}\times 6.023\times {10}^{23}=1.67\times {10}^{21}$
$\because$ 1 molecule contain 8 neutrons,
$1.67\times {10}^{21}$ molecules will contain $=1.67\times {10}^{21}\times 8=1.338\times {10}^{22}$ neutrons.