the number of octohedral sites per sphere in a fcc structure is

a)8 c)2

b)4 d)1

Let the number of atom present per unit cell is N. 
Then Number of octahedral voids = N 
Number of tetrahedral voids = 2N

As in FCC the number of atom present in per unit cell is 4. 
Therefore number of octahedral voids are also equal to 4. 
Hence, the correct option is b.