(A) confuses sufficiency and necessity and changes “most” to “all.”

We can validly infer that most single-party countries have weak local governments. (A) is a backward version of that claim with “every” in place of “most.”

This appropriately puts some before all:

Some plurality countries are corrupt. All corrupt countries are weak. Therefore some plurality countries are weak.

It’s worth noting also that “some” claims are reversible.

This could be false. A world in which (C) is false is one where all weak countries are corrupt. That’s us negating (C).

The stimulus gives us the converse of that claim. It tells us all corrupt countries are weak. Those two claims are consistent with one another – they can both be true at the same time.

That is, just because all corrupt countries are weak, that doesn’t mean there are some weak countries that aren’t corrupt.

(D) is a backward version of one of the valid inferences. As shown in the analysis section, we can validly infer “most single-party countries are weak” from the premises. (D) says that in reverse.

We can dismiss (E) for two different reasons, both of which are common LSAT themes. First, our premises only give us information about proportions, whereas (E) compares numbers. Even if a higher proportion of single-party countries are weak than multi-party countries, but that doesn’t mean the number of weak single-party countries is higher – there might be way more multiparty countries in general.

Also, “some” and “most” claims can both technically include “all”-type scenarios. That is, the claim that “some multiparty countries are weak” leaves room for the possibility that all multiparty countries are weak. So there’s not even a guarantee that a higher proportion of single-party countries are weak than multiparty countries.