The packing efficiency in a face – centred cubic cell system of crystals is 

Given below is the packing efficiency for a face-centered cubic unit cell under the premise that atoms are in contact:
Four atoms are found in each unit cell of a face-centered cubic structure. 
Total volume of unit cell =$a^3={\left(2\sqrt{2r}\right)}^3$

$\begin{array}{l}=\frac{3\pi r^3}{(2\sqrt{2}r{)}^3}\times 100\%\\ =\frac{\frac{16}{3}x\frac{22}{7}{r}^3}{8(\sqrt{2}{)}^3{r}^3}\times 100\%\\ =74\%\\ \end{array}$

Hence, the correct option is option (C) 74%.