Option A : This statement doesn't indicate whether those who use the plants as ornamentals desire shorter varieties.
Option B : At most this suggests that higher productivity is not as muchof an advantage as it otherwise would be. But there is nothing in the passage that indicates that smaller varieties would be more productive than normal-sized plants.
Option C : No reason is given for thinking that smaller varieties of plantsare more conducive to use for animal feed than are larger varieties.
Option D : Correct. This answer choice is correct because—unlike the other choices—it helps explain why smaller plant varieties could sometimes be preferable to larger varieties. A plant that isles vulnerable to wind and rain is apt to suffer less damage. This is a clear advantage that would motivate plant breeders to try to discover or produces.
Option E : This has no direct bearing on the question posed. Processed grains are not even mentioned in the passage, let alone linked to smaller plant varieties.
The correct answer is D.