One of the primary benefits of a survey is that they A. can generate cause-and-effect conclusions
B. allow efficient gathering of unobservable phenomena
C. allow for researchers to observe one's natural behavior
D. are able to provide a detailed look at a single subject

B).  Allow efficient gathering of unobservable phenomena.

Explanation:

Surveys are described as the method of gathering information from a specified group of respondents for purposes like research or assessment of the truth of thoughts or opinions associated with a process.

As per the question, one of the most considerable benefits of the survey is that 'it allows the efficient gathering of unobservable phenomena'. The data associated with phenomena like stated preferences, behaviors, beliefs, and factual information are unobservable and can not be collected through observation but surveys help in doing so as well as in the assessment of the validity or reliability of the existing propositions about such phenomena. Thus, option B is the correct answer.