Synthesis of an epoxide from 1,2-diol
Thionyl chloride first reacts with the alcohol to form an alkyl chloro sulfite,which gives various stereochemical products as shown here. 



Extending this to the current question , alkyl chloro sulfite 2 is formed.${Cl^-}$ attacks in a ${S_N^2}$ mode to give 3.

Base deprotonates 3 to give 4.

Alkoxide and chloride in 4 are in correct trans configuration for a ${S_N^2}$.This results in 5(B in your scheme).



If we take C in your question , and follow the scheme above, 9  is formed.In 9 alkoxide and chloride are not in correct configuration for ${S_N^2}$.Therefore 
C in your question does not undergo epoxidation.