Final answer:
The study design in question is a cross-sectional study, which observes a population at a single point in time to identify associations between characteristics and conditions.
Step-by-step explanation:
The study design described in the question is an observational study, specifically a cross-sectional study. This study design involves observing a specific population at one point in time to examine the prevalence of certain characteristics, such as diet soda consumption, and conditions like high blood pressure. The aim is to find associations between these characteristics and the condition of interest without manipulating the study environment or variables.