Maria purchases a premium brand of coffee at the grocery store for $8. which would be considered a substitute good?

A substitute good is one that can satisfy the same need as another. In this way, in the eyes of the consumer, the substitute good can replace the function of another, whether or not they are similar in terms of their characteristics or price. Substitute services behave in the same way. We consider that one good is a substitute for another when it is able to satisfy the same need of the consumer.  

Given the above, some substitute goods for Premium brand of coffee are 1. Value brand of coffee 2. Tea in any of its flavors 3. Water 4. Soda 5. Cocoa.