Final answer:
To expand his handmade dog collar into a product line, Marco could create matching dog sweaters and rain booties, thereby offering more coordinated items to his customers.
Step-by-step explanation:
The question is asking how Marco can expand his single product into a wider range of products, or a product line. To transform a handmade dog collar into a product line, Marco could create additional related products that complement the collars, such as matching dog sweaters and rain booties. This would broaden his offerings and likely attract more customers interested in purchasing coordinated items for their pets.
However, the remainder of the options provided seems to be irrelevant or incorrectly included, as they pertain to legal concepts such as strict liability, breach of duty, and causation, which are not relevant to creating a product line in a business context.