According to the given information :
A, B, C, D, E and F are sitting around a circular table facing the centre.
1) A is sitting second to the right of D, who is third to the left of B.
2) If C is NOT sitting with D then C will be sit to the just right of B.
Nothing is mentioned about E and F then there may be two cases.
Case 1 :
Case 2 :
So, from both cases :
'D' is sitting between E and F.
Hence, the correct answer is "D".