Sales promotion refers to short-term incentives offered to stimulate immediate interest and purchase. Free samples, coupons, contests, discounts, and rebates are all common sales promotion techniques.
In this case, Marbury Biscuits distributed free samples — a classic example of sales promotion. It is not advertising (which involves paid messages through mass media), not personal selling (face-to-face persuasion), and not public relations (managing the brand's image).
Final Answer: (C) Sales Promotion