I was stuck between C and E and ultimately chose C, even upon blind review. 

Here's why:

“ this practice is legitimate “  is the position it attributes to the proponents.

“ it helps shield against injustice ” is the premise in support of that position

“they too often make serious mistakes” is the attempt to show the falsehood of the premise that “ it helps shield against injustice ” 

Therefore, the argument is using the technique of attempting to show that a premise put forward in support of the position is false, answer choice C. 

Even if the doctrine could feasibly help shield against injustice in the presence of mistakes, the argument is still saying that “too often” it makes mistakes, meaning that the arguer is intending the making of mistakes to be at odds with the shielding agains injustice.

Even if this technique is questionable it very much seems that it’s the technique the argument is employing. 

 For answer E, yes the argument ends by saying the application of the doctrine has undesirable consequences, but the immediately preceding sentence( “But the doctrine relies excessively on jurors' objectivity” ) seems to be calling out an issue with the doctrine itself, and the following sentences is adding additional support. So it’s true that the argument is employing technique E, BUT it appeared to be a slightly less strong answer than C 

What am I missing here? For E, I definitely overthought “the application” and the meaning of undesirable consequences, but I’m really struggling to understand my error with C. 

Is it that “ it helps shield against injustice ” is not premise in support of that position, but merely a part of that position? If so how can I apply this to future stimuli? Should I assume that if something is one single sentence, it is an either a premise or a conclusions, and that a premise and a conclusion but be separate sentences?

I understand that I’m wrong and surely ’m overthinking parts of this, but what I’m really asking is HOW do I apply the fact that E is correct and C is  to a future problem?