Answer:
Marketing channels.
Explanation:
Marketing Channels is one of the core aspects of marketing which enables a company to marketize its products, services, and brand identity to it's customers.
The Gluten Free Bars (GFB) are the vegan bars created by Marshall and Elliot Rader (brothers). The brothers created the GFB in 2010, and heralded the gluten-free industry.
To marketize their products, they used the element of marketing channels. They first reached to their possible customers by shopping their products to independent coffee shops, then to grocery and natural food stores. They used the channels of independent coffee shops, grocery and natural food stores to marketize their products and services to their possible customers.
Thus the correct answer is that they used marketing channels aspect of marketing.