Final answer:
After defining the problem, Walmart marketers should next complete the research design stage of the marketing research process before data collection.
Step-by-step explanation:
Walmart marketers, after completing their preliminary work such as defining the problem and identifying the resources available, should move to the next logical stage of the marketing research process. Given the options provided, the next step they should undertake is c) Research design. This step is critical before data collection, as it involves planning out the approach for collecting data that will be most effective in solving the defined problem. Once the research design is adequately planned and prepared, Walmart marketers can move forward to collect data, which would then be followed by data analysis, and ultimately, report writing to communicate the findings.