Answer:
d. pairing all existing nonsense syllables with only positive words
Explanation:
As it is stated clearly in the description of the experiment that there are certain pitfalls or limitations. One of them provides that the syllables themselves might be unpleasant without commenting on the nature of the word/image/experience associated with it.
Now, it is clear that the limitation is directly associated with the syllable, and with its words/images.
Thus, it shall be clear that the association of such syllable shall not make any difference even if they are connected with some positive words only, as the fault cannot be corrected by connecting it with the stupid syllable.