Final answer:
The research question 'determining if parents who frequently hit their children have more aggressive children' is suitable for a correlational study. This type of research can establish if there is a relationship between two variables but cannot determine causation.
Step-by-step explanation:
The research question that must be tested by a correlational study is 'determining if parents who frequently hit their children have more aggressive children'. Correlational research aims to identify the strength and direction of relationships between two variables, which in this case would be the frequency of hitting and the level of aggression in children. The crucial aspect of a correlational study is that while it can show if there is a relationship between two variables, it does not prove causation.
In cases A, B, and C, the research questions aim to determine causality (e.g., Does reinforcement promote superior learning? Do children with ADHD respond best to a certain medication?); these questions may require experimental research where the researcher has more control over variables and can test for cause and effect. In contrast, option D, which is about room temperature affecting personality, does not clearly lend itself to either correlational or experimental methods without further context, making it less fitting for a correlational study based on the information given.