Final answer:
The most challenging step of the five-step approach to marketing research is problem definition.
Step-by-step explanation:
The most challenging step of the five-step approach to marketing research is B) Problem definition. In this step, the needs of potential customers are investigated, potential competitors are identified, and constraints on the design effort are established. The problem definition process results in a clear understanding of the scope of the design project and the resources available to solve the problem.