Yes that is accurate. The question is simply asking you to pick which out of the options is a necessary assumption. It is not suggesting that there is only ONE. There are many necessary assumptions for almost any argument. Another necessary assumption that the argument commits to is that the plants which MAY be developed into medicine, WOULD be useful for development into medicine (...what if it turned out that these plants which are "likely" to be medicinal are in fact NOT... then we could not conclude that "if the tropical rainforest is not preserved, important types of medicine will never be developed"). The necessary assumption can range to be even more general and can range to be assumptions that are perfectly reasonable. There are, in a sense, an infinite amount of necessary assumptions for any argument. When picking an answer, we are always only ever looking for ONE of them. The thing that will distinguish the incorrect answer choices is that they are decidedly NOT necessary assumptions.

I believe that high priority appears next to a question when you have answered it incorrectly both on your timed and BR attempts. Low and very low priority appear when you have, at some point, answered it correctly. You can think of it as a message to you, as a test taker, personally. These questions, in other words, should be high priority for you, because you missed it on the first go through.

Also, I think that "Psg" is meant to refer to the difficulty of parsing the passage.