The number of unit cells in $58.5 \mathrm{g}$ of $\mathrm{NaCl}$ is approximately

The molar mass of NaCl can be calculated as a sum of Na and Cl atoms. So,

$\text{M=}23+35.5=58.5 \mathrm{g}$

This means that $58.5 \mathrm{g}$ of NaCl constitutes as 1 mole, containing $6.023\times {10}^{23}$ molecules.

NaCl has a fcc unit cell, meaning it contains 4 molecules per unit cell.

So, 1 mole of $\mathrm{NaCl}$ will contain $\frac{6.023\times {10}^{23}}{4}\simeq 1.5\times {10}^{23}$ unit cells.

Therefore, the correct answer is B).