The parliament enacted the legislation

In this sentence, you want to make it clear that the legislation changed private contracts. In choices (B) and (C), the gerund (nullifying and changing) is not specific enough; it could describe either the legislation or the parliament. Conversely, "which" must refer to the preceding noun, which in this sentence is "legislation." "Which" thus makes it clear that what follows is describing legislation. Eliminate (B) and (C).

In choice (D), the phrase "which nullifies clauses" is a modifier. That means that, if the modifier is removed, the sentence should still make sense. However, If the phrase is removed, the sentence reads, "The parliament enacted the legislation in private contracts that stipulated payments in gold..." This version of the sentence does not make sense and can be eliminated.

In choice (E), the parallelism between the hyphens (to change, to nullify, to stipulate) changes the original meaning of the sentence. The sentence needs to say that the legislation changed private contracts by nullifying clauses. A list of 3 actions does not show that relationship.

In choice (A), the entire phrase between the hyphens is a single modifier. The sentence makes sense when it is removed: "The parliament enacted the legislation over the heated opposition of many legislators." Also, the modifier correctly expresses that the legislation changed private contracts by nullifying clauses.

Answer choice (A) is correct.