Answer:
Option A
Explanation:

From this figure
(AC)2=(AB)2+(BC)2
(AC)2=a2+a2=2a2
Also, (AD)2=(AC)2+(DC)2
(4r)2=(2a)2+a2
16r2=3a2
r=√34a
Now, when Na metal crystallise in bcc unit cell with unit cell edge, a= 4.29Å
We have the formula for radius
i.e, r=√34×4.29 Å =1.86Å