Final answer:
The research scenario that ensures no harm to participants is an observational study on diet choices.
Step-by-step explanation:
Out of the given options, the research scenario that ensures no harm to participants is a) Observational study on diet choices.
An observational study involves the researcher observing and collecting data on naturally occurring phenomena without manipulating any variables. In this case, the researcher would observe and record the diet choices of participants without causing any harm.
For example, the researcher could collect data on the types of food participants consume and analyze the relationship between diet choices and health outcomes.