Final answer:
Firms conduct concept testing predominantly with their target customers to validate new products and incorporate their feedback before launching to the wider market.
Step-by-step explanation:
Firms conduct concept testing for new products primarily with their target customers (C). This process helps firms gather feedback and insights on a new product's potential success in the market before full-scale production and marketing begin. Target customers provide valuable input on the product concept, which can lead to improvements, adjustments, or even rethinking the product offering altogether. Concept testing is not typically done with suppliers, employees, manufacturers, or competitors as these groups have different roles and interests in the product's lifecycle. The core idea is to validate the product with the end-users who are most likely to purchase it.