If (A) were true, this helps establish that the price of halibut will indeed go down. Price is a function of both supply and demand. Although we have evidence that supply will down because of the new lower ceilings, it’s not known whether the demand might also go down. (A) establishes that there won’t be a substantial decrease in demand. If supply goes down, but demand does not substantially go down, that provides further reason to think price of halibut will go up.

(B) doesn’t strengthen, because the negation of (B) would actually be more helpful for the argument. If there is actually NO connection between supply of halibut and demand for it, then that suggests the decrease in supply from the new lower ceilings can’t be expected to change the demand for halibut. So there’s additional reason to think that the price will go up, because we have at least some reason to think the demand might not change. We actually prefer an answer that says there’s NO connection over one that says there is a connection between supply and demand.

This doesn’t strengthen, because what matters is not whether the lost “production” of halibut will be replaced by other fish, but whether CONSUMERS will replace halibut with other kinds of fish. Even if the lost production of halibut is not replaced by increased production of other fish, that doesn’t tell us that consumers aren’t replacing demand for halibut with demand for other fish.

This doesn’t strengthen, because the negation of (D) arguably helps the argument more. If the new restrictions will NOT affect demand for other fish, then we have less reason to think that consumers will replace halibut with other fish. So we’d have less reason to think that demand for halibut will go down. That’s why we’d actually prefer an answer that says demand for other fish WON’T be affected by the restrictions over one that says they will be affected.

This doesn’t strengthen, because whether halibut is only a small or large proportion of all fish consumed doesn’t establish anything about the relationship between supply and demand for halibut or what will happen to halibut price.