You just saw a billboard that states “85% of doctors use our product! ” what technique has the advertising company used to persuade you to get the product?

Ethos, because this advertisement is using appeal. It is stating that it is more reliable and honest as to where pathos appeals to emotion and there is no emotion in this. And logos is an appeal to logic or reason, giving details and facts of the product, where as, this statement you have has nothing more than a simple statistic of how many uses it, but not what it contains or does to help.

A technique called 'social proof' would be applicable here.

Here's how it works:
Advertisers know that we go to certain businesses (barber, restaurant, mechanic, etc.) simply based on our friends' recommendations AND we’re just as likely to buy something because it’s gotten the stamp of approval by someone we know and admire.

Just look at Facebook and its snowball “liking” effect. Even saying something as simple as “Nine out of 10 people choose Tide” or “The majority of people prefer Cheerios cereal” works exceptionally well to influence human behavior.

That is super long dude