This is a weaken question. The argument is that owners of certain properties who wish to avoid development on those properties should take advantage of conservation easements, which prevent development, and give the owners substantial tax benefits. The argument assumes that, as a way of preventing development, the tax benefits of an easement are preferable (probably in dollar terms) than simply renting the property and continuing to own it. Consider each choice:

(A) This is irrelevant; all trusts that purchase lands under easements would achieve the same end regarding development, whether they re-sell the property or not.
(B) The population density of an area has nothing to do with the argument; presumably, most areas have low density before they are developed.
(C) The argument isn't about what owners can do, it's about what they should do--the fact that they can split up their land does not mean that's the route they should take.
(D) This is correct. If more financial benefits accrue to owners who rent property than those who donate an easement, those owners prevent development (as desired) and get more money.
(E) The argument is concerned with the benefits of specific land owners; the effect on other lands based on easement decisions is out of the scope of this question.