Answer
See the answer below.
Work Step by Step
a) From the figure it's a body-centerd cubic lattice.
b) $8\times 1/8=1$ in the vertexes and one in the center, 2 in total
c) Diagonal of the cube: $l.\sqrt3=4.r\rightarrow r=l\times \frac{\sqrt3}{4}$
$r=137\ pm$
