The stimulus describes how legal rules operate. It starts by stating that legal rules classify people and actions into certain categories, with legal consequences associated with each category. Thus, applying these rules to a specific case means deciding if a case's facts fit those legal categories. Such a decision ultimately decides the "legal effect" of what happened, and not "any matter of fact."

If you're trying to find a conclusion in this stimulus, you might notice that while there is a "therefore" in the third sentence, the fourth sentence seems to stand on an equal footing with that sentence — it expands on the third sentence rather than supporting it per se. But since this is a Most Strongly Supported question, we're not too interested in the conclusion/support structure of the stimulus. We can treat the whole thing as potential support for the correct answer choice.

Since it's hard to know what specific inferences we're looking for before looking at the answer choices, it can be useful on the first go-around to establish what cannot be inferred — anything that is ruled out according to the stimulus. Notice that the last sentence uses some strong language: legal decisions establish "legal effect" rather than any "matter of fact." So there's a contrast here between "legal effect" and "matters of fact." The stimulus seems to assume that these are not the same thing — i.e., it rules out the possibility that legal effects are matters of fact.