Answer: D) Making guesses
Explanation: The situation mentioned in the question about Leslie doing her study on remotely-living people and adding the unknown information on the basis of guessing technique. This step is threat to the research because she is filling-up the information by guessing, her guesses have chances of being incorrect which will lead to defects in the result of the study.
Other options are not threat because choosing subject, observation and interviewing are the correct steps that should be taken during any research. Thus , the correct option for the question is option(D).