Hey Mundasingh:
Thanks for the invite to weigh in! This is a fantastic example of why it's so critical to identify the conclusion of an argument before attacking the answer choices - a task that I don't think is typically taught all that well. While the author definitely does talk about talented managers typically NOT being assigned to development projects, his argument is that they SHOULD BE assigned to them. Ultimately, there are three ways to find a conclusion in one of these arguments:
THREE WAYS TO IDENTIFY THE CONCLUSION
1) Conclusion language (such as "therefore", "thus", "in conclusion", "so", etc.)
2) A call for action ("We should", "they must", etc.)
3) The effect of a cause-effect relationship (It is raining, so the parade will be cancelled. The rain causes the cancellation, so "the parade will be cancelled" is the conclusion)
3a) To better exemplify this, try the "Why Test". Facts in these arguments don't have a reason why. Why is it raining? It doesn't say. But it does give a reason "why" the parade will be cancelled - because it's raining. So "the parade will be canceled", because the argument provides a reason why, is a conclusion based upon existing facts.
So...given all that, the conclusion of this argument is "the best managers in a company should be assigned to development projects" - it has a call for action "SHOULD BE", and it passes the why test. Why should they be assigned to those projects? The passage says because "early in projects the critical resource is talent"; because "new ventures require a disproportionate degree of managerial skill"; etc. So the conclusion is "the best managers should be assigned to development projects".
B weakens that, giving a reason that they shouldn't be (it's a small portion of the company's operations).
C weakens that, giving a reason that they shouldn't be (they probably don't want to)
D strengthens it, giving another reason why they should be (we already know that they'll significantly improve the odds of the new project's success; D says that they'll also not really be missed in the regular operations)
I hope that helps - keep in mind that the conclusion is by far the most important part of any of these arguments, so take care in making sure that you identify and embrace it.