Answer:
A. A professional review
Explanation:
Let's use process of elimination:
A) A professional is least likely to be biased, unless they've been paid.
B) An advertisement is by the manufacturer, and thus more likely to be biased.
C) Online testimonials can also be biased, because most of them are paid.
D) The seller only wants to make money. They're going to be the most biased of the bunch, as it will hurt their chances of making money if they give you a bad opinion.
