Here, I selected: Subtle and understated; did not know nebulous meaning
But even if I knew the meaning of nebulous- I could not point to any evidence for actually either of the two choices- I just know that something is difficult- now difficult because subtle or because imprecise, that I could not figure.
Using math strategy, we see the comma after the blank, which indicates support. Specifically, the phrase after the comma says that stereochemistry nomenclature “combines concepts that should be kept discrete”.
Since discrete means separate or distinct (a definition that can also be inferred because discrete is used as a contrast with combines), we now know the difficulty with the “relevant nomenclature” is because so many concepts are unnecessarily merged together, making them imprecise and nebulous.