6.
Your Answer is
Correct Answer is H
Explanation
According to the meaning of the sentence, Blades thinks that this reform will become possible...so the attributive clause should be used here to modify reform, excluding F/G.
Item J: Although it is an attributive clause, believing (not a predicate verb) cannot be used as a predicate of Blades, so it is wrong.
Item H: which modifies reform, the subject in the object clause guided by believe in the clause, that is, Blades believed (the reform) would be possible only if...