Aster Inc. has developed a new digital three-tier food steamer. Though the product comes with a self-explanatory manual, the controls and the operation of the appliance have to be explained to the customer on a one-to-one basis, in great detail. Which of the following elements of the promotional mix is Aster most likely to rely on to sell its products?

a. Advertising
b. Sales promotion
c. Public relations
d. Personal selling