Answer :
Answer:
D). They have not defined happiness or how it will be measured.
Explanation:
As per the question, the most consequential flaw or error in this study design is that 'Jean-Gabriel and Magali have not defined happiness and neither provided a description or method that describes how it will be measured.' This flaw would make the design ineffective as it would not be able to produce reasonable deductions or conclusions because they have decided that 'they would visit a mall to survey people of different ages' but this requires a scale on which they would measure happiness. Since they have not mentioned any such scale which implies that they won't be able to collect data and hence, the study could not be taken forward. Therefore, option D displays the major error of this design that makes it invalid.