Past perfect is used to indicate something that happened before something else – and because we are only given one sentence on SC, that other thing also needs to be in this sentence.

It might seem like we are saying “they used to be allowed to dump X amount and now they can dump Y amount” – two different things, right? But the sentence doesn’t actually spell that out. It just mentions the one event. We don’t have a second event or activity that occurs after… so we can’t use past perfect.

Present tense (“are”) is appropriate to use here because it tells us that the state of affairs is still true today, so we know D must be right.