Answer:The primary research that will help you to know the things you don't know is option a. "OBSERVING TARGETED CUSTOMERS INTERACTING WITH A PROTOTYPE.
Explanation: a prototype is a design work, but not the main design, made to understudy how the proposed design will work if created.
In any study, the first thing to do is to observe, because from observation we learn the questions to ask, and the inquiries to make.
This is the best option, because observing how the customers interact with the prototype, is just like observing how the Interaction with you and the customer be, if you start the business, so this will tell you more about the business, and the aspect of the business you should strengthen more, to be able to satisfy the needs of the customers.