The answer to this question would be G, 3' hydroxyl
The reason it is G is because of the rule: C-G, A-T. You can look up the structures to see that this rule reflects the fact that: a) one must be pyrimidine and other purine b) able form the same number of hydrogen bonds (2 for AT and 3 for CG) so that the two strands can align together properly
The reason for 3' hydroxyl is because two strands are antiparallel: If one strand has 5' at one end, then other strand has 3' at the same end. Also, phosphate is at 5' C of sugar and hydroxyl is at 3' C of sugar.
Hope that helps!