Keisha Received a Stiff Cardboard Mailer That Contained a Free

Keisha received a stiff cardboard mailer that contained a free sample of Advil PM, a pain reliever formulated to help people sleep. The mailer also included a certificate for $2 off the purchase of the over-the-counter drug. The manufacturer of Advil PM is most likely using a:

A) rebate.
B) spiff.
C) coupon.
D) trade promotion.
E) P-O-P material.