Final answer:
Mindy's study is a cross-sectional study, as it compares different segments of the population at one time to observe how a vegan diet correlates with happiness levels.
Step-by-step explanation:
Mindy's research, which asks vegans and non-vegans to complete the subjective happiness scale and compares their answers, is best classified as a cross-sectional study. This type of study compares different segments of the population at the same time to see how certain factors, like diet in this case, are associated with outcomes, such as happiness levels. It is not a controlled experiment because Mindy is not manipulating any variables or assigning participants to different groups herself. Instead, she simply observes and compares the existing conditions of the groups. Moreover, it is not a longitudinal study since it does not follow individuals over a period to see how their happiness evolves. Lastly, while it shares characteristics of an observational study, in that Mindy is not intervening in the participants' diets, the fact that she's specifically comparing different population segments at a single point in time rather than watching how variables unfold naturally makes the term 'cross-sectional study' a more accurate description.