Final answer:
A "salvage operation" in an unplanned multi-stage research is used when the second research strategy helps explain unexpected or puzzling results, preserving the study's integrity. (Option D).
Step-by-step explanation:
When unplanned multi-stage research is described as a "salvage operation," it typically refers to situation b) when the second research strategy is used to explain unexpected or puzzling results.
This scenario occurs when the initial research strategy fails to account for all outcomes or when results defy the predicted outcomes.
This necessitates a change in the research approach to salvage the study and further investigate the unexpected findings. Retesting is important in scientific research and social studies as it helps to eliminate errors and validate the range of accuracy in findings.
The goal of retesting and employing a salvage operation is to maintain the integrity and validity of research studies. Therefore, the correct option is D.