Answer:
a) Direct retailing
Step-by-step explanation:
The direct retailing is the process of direct selling the company products and services via door to door, office to office from company to customers rather purchasing online, store locations, etc
According to the given situation, Delta Graphics Inc. specializes in publishing educational books. And, in order to sell their products, the company recruits and trained college students so that it sells the company products door to door by using their convincing skills, product information, product comparison, etc