The process used by the fast-food hamburger restaurant is called test marketing.
Test marketing refers to a marketing process that involves the study of customers' responses toward a product or a service. It is done by making the product available at limited marketplaces before making a wider release.
In the given case, the fast-food hamburger restaurant made available the newly released product, pizza, to the established market to study the consumer reactions. This was done with a view to obtaining knowledge about the preference of the customer.
Therefore the correct option is test marketing.
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