Final answer:
During the systematic marketing research approach, marketers identify the data needed for marketing actions in the stage of developing the research plan.
Step-by-step explanation:
The stage of the systematic marketing research approach during which marketers identify the data needed for marketing actions is Developing the research plan. This stage involves outlining the specific information that needs to be collected and the methods that will be used to gather that data. Marketers may utilize various techniques such as survey questionnaires, interviews, observations, or analyzing existing data sources to gather the necessary information. For example, a marketing manager might develop a research plan to gather data on customer demographics by distributing questionnaires to randomly selected customers at store locations.