ANS: B - (1/8)
This is a simple division series; each number is one-half of the previous number. In other terms to say, the number is divided by 2 successively to get the next result.  

4/2 = 2
2/2 = 1
1/2 = 1/2
(1/2)/2 = 1/4
(1/4)/2 = 1/8 and so on.