Answer:
Cross-sectional research method
Explanation:
The Cross-sectional research method is a method of observational study design. Here, in this study the experimenter or the researcher measures and studies the outcome and and exposures in the study participants in the same time.
In the context, An experimenter studies the heredity and the intelligence of the MZ twins who are reared together and for those who are reared apart. The findings of the experiment made by the researcher is a result of a cross sectional research method in this case.