(E) "rejects a claim merely because the person making the claim stands to benefit by doing so"
Let's make sure we understand what this answer choice means. Here, Sabine is the person rejecting the claim that the herbal supplement helps prevent baldness merely because Kevin's barber stands to benefit by making the claim. How does Kevin's barber benefit? By making money from selling the herbal supplement.
If we look at the passage, we see that this is exactly what Sabine does. Sabine discounts the claim immediately because there was motive to make it. However, we know that this is illogical. Just because someone has the motive to make a claim, does not mean that the claim must be false.
Does that make sense? Please let us know if you have any other questions!