In Sarminia the quantity of wheat produced per year is currently just

Premise:
1. In Sarminia the quantity of wheat produced is just large enough to satisfy domestic demand.
2. Total wheat production of Sarminia will remain same in near future.
3. Sarminia's population, however, will be increasing significantly for years to come.

Conclusion: Therefore, Sarminia will soon have to begin importing wheat.

We need to weaken the conclusion. What if the the demand for wheat reduces with the increased population i.e., the increased population does not eat wheat anymore? There would be no need to import wheat and the conclusion is weakened.

A. At present, no one in Sarminia is in possession of an import permit authorizing the importation of wheat into Sarminia.
--> This just means that with the increased demand for wheat, the import permits will have to be issued to allow import of wheat. This does not weaken the conclusion.

B. It is unlikely that Sarminian importers could obtain wheat from a foreign source at substantially lower prices than those charged for domestic wheat.
--> So the Sarminian importeres will have to pay more to import wheat. The question stem does not say that Sarminian's will not be able to pay more to support the increased demand.

C. The proportion of Sarminian families who produce all or most of their own food has been shrinking for years as Sarminia's population has become increasingly urbanized.
--> This statement supports the conclusion instead of weakening it. If local produce is shrinking, all the more reason to import wheat to support the increased demand.

D. Sarminia's current population growth is largely due to the influx of immigrants from Sarminia's western neighbor, and in these immigrants' diets rice is the grain staple.
--> If the increased population does not need an increased quantity of wheat in their diet, then there is no need to import the wheat. This weakens the conclusion.

E. In Sarminia existing facilities for the storage of wheat have little spare capacity beyond what is needed to accommodate Sarminia's own crop.
--> This statement implies that new storage facilities will have to be setup for wheat imported to support increased demand. This does not weaken the conclusion that wheat import is needed to support increased demand because of increased population.

Answer: D