Final answer:
An open-ended question is most suitable for the study on pre- and post-retirees, allowing in-depth responses and qualitative insights.
Step-by-step explanation:
Based on the given scenario regarding the study on the differences in the length of stay for pre- and post-retirees, an open-ended question would be the most appropriate to ask about the observed results of the study. Open-ended questions allow respondents to provide more detailed information, which is ideal for gaining insights into complex issues such as the different behaviors and preferences of pre- and post-retirees. These types of questions can yield qualitative data that is rich in detail and context, offering a deeper understanding of the subject matter.