This is a question from the GMATPrep so it is an official question.

As an assumption question we need to begin this question as we would for a strengthen question and then see where the gaps in reasoning are. It is actually pretty straight-forward when we do this.

First identify the main conclusion: "most other scientists conclude that this popularizer (who have written science for popular audiences) should no longer be regarded as a true colleague."

There is only one premise! So that is the evidence - or as I call it the "Most Important Premise." It is "Generally scientists enter their field with the goal of doing important new research and accept as their colleagues those with similar motivation."

Let me rephrase this: "Scientists accept others as their colleagues if those people have a goal of doing important research."

So when the conclusion says that the popularizers will not be accepted as true colleagues the reason is right there in the evidence. It must be because these popularizers do not appear to have a goal of doing important research.

This is what choice D says.

Choice A is a popular incorrect answer for this question; it appears to be necessary as it plays into the whole "colleague" discussion. However, this has nothing to do with the main evidence which is that scientists accept as colleagues those who are motivated to do important research. Solitary or in groups is really not part of the discussion. It is important research versus writing science for general audiences and becoming famous.

Some choices can be eliminated simply by asking if the condition is met. For example on B, do the scientists envy the others? Is that something that we know?

On other choices ask if it would make a difference if this were true or not. For choice C, does it matter if a scientist can become popular without important research? No. What matters is what the other scientists think are his motivations.

Finally choice E, gives a great example of the assumption answer that is too absolute. Is it really a required assumption that "NO important new research be readily accessible..."? NO and ALL are not usually correct as assumption answers since this level of statement is not required.

I hope that helps you to look at some of the ways to work with assumption questions.