Nonagricultural commodities have nothing to do with the reasoning of the argument, which concerns agricultural equipment and agricultural imports.

The ability to “enter international markets” is not what’s at stake. What’s at stake is the potential for country X to not be able to supply us with enough agricultural imports if we decide not to sell the agricultural equipment to it. So the author doesn’t have to assume anything about the ability to enter international markets.

“Never” is too extreme. The author doesn’t assume that we should never allow our interests to be threatened to punish a protectionist country. It’s just that in this specific situation — where our country demands a product that could be produced by the other country — we shouldn’t hurt the ability of that other country to produce the product just for retribution. But in other cirumstances, the author might agree that it’s OK to jeopardize our own interests to punish a protectionist country.

Not necessary, because the author is advocating that in this circumstance, we should take an action that doesn’t involve punishing a protectionist country. So it wouldn’t undermine the author’s reasoning if (D) were not true.

Necessary, because if it were not true — if there is no reason to balance the justice of an action with the consequences to our own interests — then we would have no reason to refrain from punishing country X. The author’s reasoning is based on withholding punishment in this case because of how the punishment will affect our interest in getting agricultural imports from X.