What fish are covered by the term fugu?

Does the term refer to
a) any fish of the Tetraodontidae family of fish (generically called puffer or blow fish)
b) specific gerera of this family - Takifugu, Lagocephalus, or Sphoeroides (ref wikipedia)
c) Only the Takipgu genus and its 25 off species
d) only the specific species takifugu rubripes (tiger puffer)

How do the Japanese actually understand and use the word

Thanks

I don't know what you are trying to find out, but here is my take as a Japanese.

When I think of "fugu" as seafood, the only names I have seen in fishshops/restaurants are: "torafugu" (Takifugu rubripes) and "mafugu" (Takifugu porphyreus). Maybe some other gourmets among the locals here know more, but that's all I know.

If you are talking about what I get to see in aquariums, by looks I would call "harisenbon" (Diodon holocanthus), "itomaki fugu" (Kentrocapros aculeatus) and "hako fugu" (Ostracion immaculatus) also as "fugu" (pufferfish/blowfish). All those three are outside the Tetraodontidae family.

It may well depend on persons and regions which animals or plants he/she regards as under a same category and what he/she calls each species and each category. (That is not unique to Japanese, I suppose.)
So, if "the Japanese" means Japanese ordinary people, we may have various answers.

Fish species which have a Japanese standard name ending with "fugu" seem to be of at least 3 families under Tetraodontiformes Order [Fugu moku]: Tetraodontidae Family [Fugu ka], Ostraciontidae Family [Hakofugu ka] and Diodontidae [Harisenbon ka].
As you know, many fish under this order [Fugu moku] have one or a few toxic parts. Tokyo Metropolis (with many restaurants and consumers), Yamaguchi Prefecture (known for fugu, which is often called "fuku" there) and some other prefectures have a fugu cooking license system. The Tokyo license rule is designating 22 fish species as fugu used for food, which seem to range over at least the above three families.