Necessary, because if it were not true — if there are NO substances with medicinal value in tropical rainforest plants that haven’t been studied yet that are different from substances we’ve already discovered — then we have no reason to think that we’ll be unable to develop important types of medicine if the tropical rainforests disappear. In other words, the negation of (A) opens the possibility that all of the plants we haven’t studied in a tropical rainforest don’t have any new or different medicinal value from stuff we already know about. If that’s the case, the author hasn’t shown we need to preserve the rainforest in order to develop certain important types of medicine.

Not necessary, because the argument is arguably strengthened if (B) were not true. If half or more of the tropical rainforest plants CANNOT be found in other types of environment, that raises the importance of preserving the tropical rainforest for the purpose of preserving access to those plants.

Not necessary, because even if most plants unique to the rainforest haven’t been discovered to contain substances of medicinal value, as long as there is at least one kind of unique plant that hasn’t been studied that has medicinal value, the author’s reasoning still works. The author doesn’t argue that most of the unstudied plants will be valuable; only that there’s at least some that will be.

Not necessary, because the author’s assumption is not that IF we preserve the rainforest, then we will find every substance of medicinal value. Rather, the author thinks that if we DON’T preserve the rainforest, then at least some subtances with medicinal value we won’t be able to find. In other words, the author assumes that it’s necessary to preserve the forest, not that preserving the forest and studying the plants there is sufficient to find plants of medicinal value.

Not necessary, because what “should” happen is not required by the argument. The argument concerns whether preserving the forest is necessary in order to develop certain medicines; whether the forest “should” be preserved is a separate issue.