Missed the word IMMEDIATE before successors. Blah. Frustrating way to get one wrong.
Immediate successors doesn't refer to his son Adam who came later but rather to those who came after his death in 1935 and unlike Parry emphasized the fixed nature of what oral poetry can and can't do (lines 38-44) found precisely in answer choice D emphasized the boundaries of oral poetry.
The difficulty level of this is like a one -- shocked and appalled I got this wrong. Oh well moral of the story is read every word very carefully or else you'll make a silly mistake come test day.