D is immediately behind A i.e. A, D.
D and C are fourth and fifth respectively i.e. _, _, A, D, C, _, _.
There is exactly one person between C and G i.e. _, _, A, D, C, _, G.
F is behind B, D and E. So, F is between C and G while the first two positions are taken by B and E.
Thus, the sequence becomes:
B/E, B/E, A,D, C, F, G.
Clearly, F is sixth in the line.