When an ad suggests that everyone except for you is using the product, they are using the _ _ technique.
a. put down
c. bandwagon
b. hidden fears
d. testimonial

They are using the bandwagon technique - C. 

If they used customer testimonials about how certain customers liked this product and you could see their actual testimonials, then D would be the answer. However, given the fact that the ad suggests that everyone uses this product except you, they're relying on the bandwagon technique.