Why are Justify Questions Described in Terms of Sufficiency?

The sufficiency model explained here reveals the abstract nature of the correct
answer choice in a Justify question.
Earlier in this chapter we noted that the correct answer to a Justify the
Conclusion question is sufficient to prove the conclusion. This is because Justify
questions all conform to the following relationship:
Answer choice(Correct)------------> Conclusion(Valid)
As we will see when looking at Justify question stems, the language used by the
stem will convey that the answer choice is sufficient to prove the conclusion, as
in the following stem:
“The conclusion above follows logically if which one of the following
is assumed?”
Using the conditional indicators in the stem, we can rearrange the stem as
follows:
“If which one of the following is taken as true (assumed), then the
conclusion above follows logically?”
From a conditional standpoint, this shows that the correct answer is sufficient to
make the conclusion follow logically. This does not mean that the answer
choice must contain a sufficient condition indicator or that the argument must be
conditional in nature (although many are)! The sufficiency model is a way of
showing that the correct answer choice will add enough to the argument to
make the conclusion follow.