Let's diagram the principle. We know "unless" introduces the necessary condition, so our necessary condition here would be "good reason to think that their interests substantially coincide with those of the advice giver in the circumstance in question" (aka "ISC"):
===> ISC
We would then negate the other part of the sentence for our sufficient condition so "regard the proferred advice with skepticism" would become "not regard the proferred advice with skepticism" (aka not S):
not S ===> ISC not ISC ===> S
There are only two ways to properly invoke this general principle:
(1) invoke "not S" to conclude "ISC"
OR
(2) invoke "not ISC" to conclude "S"
Just based on this we can eliminate (E). (E) is concluding "should follow his recommendation" (aka not S), but that is the sufficient condition of our general principle. Remember, we invoke the sufficient condition to conclude the necessary condition.
Now let's turn our attention to (A), (B), (C) and (D).
Answer choice (A) tells us that because Floyd is the sole owner of R&M, the company he just recommended Harriet invest in, Harriet should reject his advice out of hand and invest her savings elsewhere. (A) is invoking the sufficient condition of the contrapositive, i.e. interests do not substantially coincide, but the problem with (A) is that we are not concluding that Harriet should be skeptical of this advice. Rather (A) concludes that Harriet should reject this advice out of hand. As such, (A) is incorrect.
(B) tells us that the salesperson, whose commission increases with the price of the refrigerator sold, warned Ramon against the least expensive model. Clearly, Ramon's and the salesperson's interests do not substantially coincide in this circumstance. On the basis of this, (B) concludes that Ramon should not reject the least expensive model on the salesperson's advice alone, i.e. he should regard the salesperson's advice with skepticism. Notice that (B) is invoking the sufficient condition of the contrapositive, i.e. "not ISC" to conclude the necessary condition, i.e. "S." Therefore, (B) is the correct answer.
(C) is incorrect because Mario's and Yvette's interests here substantially coincide, i.e. they both prefer these chocolate fudge brownies to any other pastry. "ISC," however, is the necessary condition of our general principle so it tells us nothing us. Therefore, (C) is not a valid application of this general principle. Again remember that we invoke the sufficient condition to conclude the necessary condition.
(D) concludes that "Ron should investigate further before deciding whether it is the best textbook for his course" (aka S) based on the premise that the editor of the book is a personal friend of Sara's. (D) is trying to invoke the sufficient condition of the contrapositive, i.e. interests do not substantially coincide, to conclude that Ron should be skeptical of this advice, i.e. he should investigate further before making a decision. But both Ron and Sara wrote a chapter of this textbook so there is good reason to believe that their interests substantially coincide here. Therefore, (D) is not the correct answer. (D) is close but (B) is the better answer because in (B) Ramon's and the salesperson's interests clearly do not coincide, whereas in (D) seems like Sara's and Ron's could.
Hope this helps! Please let us know if you have any other questions.
