Answer: Case study
Explanation:
Case study is defined as research technique that is carried out to investigate and identify the real life conditions.It consists descriptive content that explains about the phenomena that is analysed in case study.
According to the question,Krista and Tatiana are the co-joined twins which makes them a real life factors which can be studied for case study to know about their real-life circumstances .
Other options are incorrect because experiment is conducted in artificial environment. Survey is examination of something to make it in the form of record.Naturalistic observation is observing and analyzing the element of natural environment.
