The question is asking about what realists would most likely to be true about the dicta in a judge's written opinion. Without considering the answer choices, we can gather from the passage that the realists think that it is not precisely the same from judge to judge. What one judge, who sets the precedent, may think is non-essential (dicta), another judge may think is essential (holding). It is due to this variance that judges will be faced with indeterminacy because precedential cases may have conflicting applicable laws in the holding, based on disagreements between judges of what is dicta and what is not.
This is why answer choice D is correct.
Answer choice A is not supported as what the realists would think about the judges' dictas because we do not know that they think judges are typically careful in their assessment. If you look back at lines 35-40, I think you'll see that there is no indication of them explicitly stating nor implying this. All they mentioned in lines 35-40 is the degree of freedom that future judges will have in classifying holding vs dicta.
I hope this helps. Please let me know if you have any other questions!