Answer: c. a unique selling proposition
Explanation:
A unique selling proposition is when a product has a unique feature that differentiates it from other products.
Smithy writing's pens writes on dark surfaces. This feature is unique to smithy writing only as other competitors don't have this feature. This is an example of a unique selling proposition.
Product party is when a product is so similar to other products that consumers can use the product in place of other similar products.
Consumer stimulants induce consumers to purchase a product. They include promos, discounts, coupons etc
Specialty advertising is when a company gives promotional products to consumers in order to increase brand awareness.