The author concludes that engineers deserve credit for their contributions to the invention of useful devices.
Why? Because, usually, engineers must translate an inventor's insight into something workable and useful.
I say "usually" because the passage tells us that, sometimes, inventors serve as their own engineers. This tells us that engineers do not always deserve credit for their contributions because, sometimes, they do not contribute to the invention of useful devices.
This is restated by answer choice (B), making this our correct answer.
Answer choice (A) is incorrect because the stimulus does not distinguish between theory and practice. The author is focused on the practical issue of whether engineers receive sufficient recognition for their contributions to inventions.
Answer choice (C) is incorrect because the statement is not a premise; it does not support any earlier statements in the argument.
Answer choice (D) is incorrect because the distinction is not necessarily unclear. The author simply concedes that their suggestion does not apply in every case.
Answer choice (E) is incorrect because it is not an alternative solution. The author says that the problem is that engineers get too little recognition for their contributions. Therefore, the author concludes that they should get recognition.
Hope this helps! Please let us know if you have any further questions.