When we can extract permanent 6..if we need to extract it.. after formation of 7 furcation

When we can extract permanent 6..if we need to extract it: 

A. after eruption of 7

B. after eruption of 5 

C. after formation of 7 furcation *** 

D. after formation of third molar

After formation of furcation of 7, at this stage the root will be developed and these will help the tooth to move to the 6 space.