Final answer:
To explain the findings of a nicotine study, option a) 'The participants in the study had a history of smoking' is most relevant as it directly links participants' past behavior with the current effects being studied, providing a potential causal relationship between smoking and the health outcomes in question.
Step-by-step explanation:
The question posed relates to explaining the results of a nicotine study. To determine which of the provided options most helps to explain the findings of a nicotine study showing an association between nicotine and certain health outcomes, it is important to understand the context and methodologies of the study. In this case, the options suggest factors that could affect the validity of the study's outcomes, such as sample characteristics and study conditions.
Participants in the study having a history of smoking (option a) may provide a direct link to the observed health outcomes. Nicotine levels being within a normal range (option b) would indicate that the observed effects were relevant to typical exposure levels. The study being conducted in a controlled environment (option c) ensures that external variables are minimized. Finally, participants being randomly selected (option d) ensures that the sample is representative and reduces selection bias.