Unilluminating
From the author's line " ...there has been less incentive for studying them scientifically; why should one develop a theory, carry out systematic observations, or make predictions about the obvious” in the last paragraph, it is easy to deduce that attempts to treat human relations scientifically have thus far been relatively unilluminating as they are not really able to do a lot of value addition.
Therefore, answer option (a) clearly stands out as the most appropriate answer choice. Option (c) is incorrect because it is a very strong tone. We should avoid strong tones in general while answering tone based questions.