Which type of data refers to historical data structures previously collected for research problems?

The identification of secondary data as the correct answer stems from its definition and purpose within the research context. Secondary data refers to information that has already been collected and recorded by others for purposes other than the specific research project at hand. This includes historical data, statistics, reports, and data gathered from previous studies. It allows researchers to leverage existing resources to inform their current analysis, making it an efficient choice for gaining insights without the need for direct data collection.

Primary data, in contrast, is original data gathered firsthand for a specific research objective, which does not pertain to previously collected information. Tertiary data isn't generally recognized as a formal category in research methods; it typically encompasses summaries and compilations of primary and secondary data rather than the raw or historical data itself. Qualitative data refers specifically to non-numerical information that provides insight into behaviors, motivations, and feelings, which does not apply to the historical data context described in the question.

Thus, the understanding of secondary data as pre-existing historical information makes it the most fitting answer to the question posed.