Sodium metal crystallizes in a body centred cubic lattice with a...

For B.C.C lattice,

$4 \text{r} = \sqrt{3} \text{a}$

where 'r' is radius of atom and 'a' is edge length of unit cell

$\text{r} = \dfrac{\sqrt{3}}{4} \text{a} = \dfrac{\text{1.732}}{4} \times \text{4.29}$

$= \text{1.86} Å$